Easy so that hard can be hard
Most runners think of an easy run as a lighter version of a proper one. It works better to see easy runs as a separate tool with its own job: they let you train often, and they leave you fresh enough to hit your hard sessions properly.
That’s why the balance between easy and hard running matters more than any single session. With too much in the middle, every run feels like work and the hard days never get hard enough. Get the balance right and you can train more, recover faster, and get full value from each hard session.
This article covers how to change that balance through the year, whatever distance you’re training for, and how to measure it, so you know what your training looked like rather than what you planned.
If you’re in your first few months of running, you don’t need any of the structure below yet. Run everything easy, as in Couch to 5K, and come back once you’re running comfortably three or four times a week.
Counting in three buckets
Your watch probably shows five heart rate zones. For planning your training balance, it’s simpler to count in three buckets. These match the intensity domains used in our 5K session series:
| Bucket | Intensity domain | Heart rate zones | Effort (out of 10) | Typical sessions |
|---|---|---|---|---|
| Low | Moderate | Zones 1–2 | 2–4 | Easy runs, long runs, recovery runs |
| Middle | Heavy | Zones 3–4 | 6–8 | Steady runs, tempo, threshold, marathon pace |
| High | Severe | Zone 5 | 9–10 | 5K race pace, VO₂ max intervals (maximal oxygen uptake), hill reps |
The domain names can trip you up. The moderate domain is your easy running, and the “moderate-intensity trap” that runners warn about is in fact the heavy domain. For the rest of this article we’ll stick with low, middle and high.
Easy running means zones 1 and 2 only. The moment your heart rate goes into zone 3, you’re running too hard for an easy run. You’ve moved into the middle bucket, whatever the session was meant to be.
The boundaries between the buckets are your first lactate threshold and your critical speed, the fastest pace your body can hold at a steady state. The block progression run explains both.
How you spread your training across the three buckets is called your training intensity distribution. There are three common patterns:
- Polarized: mostly low, a meaningful amount of high, and very little middle. Roughly 80 / 5 / 15.
- Pyramidal: mostly low, less middle, and even less high. Roughly 80 / 15 / 5.
- Threshold-heavy: a large share in the middle. This is where most recreational runners end up without meaning to.
Polarized and pyramidal share the same base, with around 80% of the time low either way. The only difference is how the hard 20% is split.
What the research supports
Across several meta-analyses, a distribution with roughly 75–80% low-intensity work performs best for endurance.1, 2 In one well-known nine-week trial, polarized training improved VO₂ max and time to exhaustion more than threshold-heavy, high-intensity-only or high-volume training.3
The details matter for how you plan a year. Polarized training’s advantage is clearest over blocks shorter than 12 weeks; over longer periods, several distributions produce similar gains.1 Elite distance runners are mostly pyramidal. Across the season they spend a lot of time in the middle bucket, particularly at threshold, and often become more polarized as a race approaches.4
The order of the phases matters too. A 16-week study of well-trained runners compared four plans, and the one that ran pyramidal for eight weeks and then polarized for eight gave the biggest improvement in 5K time.5
Put together, the evidence doesn’t support a fixed 80/20 all year. It supports keeping the low share steady at around 80% and changing how the other 20% is split as the race gets closer: pyramidal while you build, polarized as you sharpen.
Periodizing your year, whatever the distance
Periodization simply means dividing your training into phases, each with its own job. The same five phases work for any distance, from 5K to the marathon. What changes is how long each one lasts and what the hard sessions look like.
The phases and their distribution
Figures are share of total running time: low / middle / high.
| Phase | Job | Target distribution | What the hard 20% looks like |
|---|---|---|---|
| Base | Build volume and aerobic fitness | 90+ / up to 10 / 0 | Strides and one steady or tempo run a week. No true hard sessions. |
| Build | Raise your threshold | ~80 / 15 / 5 (pyramidal) | Threshold intervals and progression runs, plus occasional short fast reps |
| Race-specific | Sharpen for race demands | Depends on distance (see below) | Sessions at and around goal race pace |
| Taper | Arrive fresh | Proportions shift toward hard as volume drops | Short, sharp reminders at race pace. Volume falls, intensity doesn’t. |
| Recovery | Repair after the race | 100 / 0 / 0 | None. See reverse tapering. |
The race-specific phase is where distance matters, because your race pace falls in different buckets:
| Race | Race pace sits in | Race-specific distribution | Key sessions |
|---|---|---|---|
| 5K | High | ~80 / 5 / 15 (polarized) | Race-pace reps, VO₂ max intervals |
| 10K | Top of middle, into high | ~80 / 10 / 10 | Reps at 10K pace, threshold intervals |
| Half marathon | Top of middle, near threshold | ~80 / 15 / 5 (pyramidal) | Longer threshold blocks, half-marathon-pace runs |
| Marathon | Middle | ~80 / 15–20 / under 5 (pyramidal) | Marathon-pace segments inside long runs |
Marathon runners often worry that their training “isn’t polarized”. It isn’t meant to be. The race is run in the middle bucket, so the specific work has to be there too. What stays the same across every distance is the 80% low share, while the 20% moves toward whatever your race demands.
How long each phase lasts
These are coaching starting points, not research findings. Shorten the base if you’re coming straight off a previous training block with good fitness, and lengthen it if you’re returning from a break.
| Race | Base | Build | Race-specific | Taper |
|---|---|---|---|---|
| 5K | 6–8 weeks | 4–6 weeks | 2–3 weeks | 7–10 days |
| 10K | 6–8 weeks | 4–6 weeks | 3–4 weeks | 7–10 days |
| Half marathon | 8–10 weeks | 4–6 weeks | 4–6 weeks | 10–14 days |
| Marathon | 8–12 weeks | 4–6 weeks | 6–8 weeks | 2–3 weeks |
If you race several times a year, you don’t need a full base phase before every race. After the recovery phase, a few weeks of mostly easy running and then a shorter build is usually enough.
How many hard sessions per week
By time, the 20% goes a long way. A 25-minute threshold set inside a 55-minute session is only a small slice of a four-hour week. What limits you is the number of hard sessions, because each one needs recovery.
| Runs per week | Hard sessions (middle or high) | Everything else |
|---|---|---|
| 3 | 1 | Easy, including the long run |
| 4–5 | 1–2 | Easy, including the long run |
| 6–7 | 2 (occasionally 3 for experienced runners) | Easy, including the long run |
Keep at least one easy day or rest day between hard sessions. A long run counts as low unless it includes race-pace segments, in which case it’s one of your hard sessions for the week.
How to measure it
You can’t manage a distribution you don’t measure, and almost everyone overestimates how easy their easy runs are. There are three measures to use, and each answers a different question.
1. Label every session before you run it
Before each run, decide what it is: low, middle or high. A session takes the label of its hardest planned part, so a threshold session with a long warm-up and cooldown is still a middle session.
At the end of the week, count them. This is the session-goal method, and it’s what controls your recovery. If you planned one hard session and ran two, your week is off no matter what the time totals say.
2. Check time in zone after every easy run
Your watch’s time-in-zone breakdown is most useful for one job, which is catching easy runs that weren’t easy. After each low session, check for any time in zone 3 or above.
The target is zero: an easy run stays in zones 1 and 2 from start to finish. Any time in zone 3 means you went too hard. Slow down sooner next time, and walk the hills if that’s what it takes to stay in zone 2. This drift is how a planned 80/20 week turns into a threshold-heavy one.
Set the top of zone 2 as a heart rate alert on your watch, so you know the moment you go over it.
If you train without a heart rate monitor, the talk test does the same job. The aerobic base guide explains how to set your zones and check them against how you feel.
Why the two numbers disagree
Time in zone and session counting give different answers, and that’s expected. In elite skiers, heart rate time-in-zone showed roughly 91% low and 3% high. Counting the same training by sessions gave roughly 75% low and 17% high.6
Time-in-zone undercounts hard work for two reasons: heart rate takes a minute or more to catch up with each fast rep, and warm-ups, cooldowns and recovery jogs all count as low. A comparison of the methods found that heart rate alone consistently underestimates the high-intensity share.7
So use each for its own job. The session count caps how many hard days you have, and time in zone keeps your easy runs easy. The target distributions in the tables above are based on time, but read them alongside your session count.
3. Track load with session RPE
Rating of perceived exertion (RPE) is how hard a whole session felt, on a 0–10 scale. Record it about 30 minutes after you finish, then multiply by the session’s length in minutes to get a session RPE load.8
It takes seconds, needs no equipment, and puts easy and hard sessions on the same scale. It’s also where drift shows up first: if your easy runs are regularly scoring 5 or 6 rather than 2 to 4, they’ve stopped being easy, whatever your heart rate says. Watch the weekly total for sudden jumps in the same way you’d watch mileage. Building weekly mileage safely covers how quickly load can rise.
A worked week
Here’s a four-run week in the build phase for a 10K:
| Day | Session | Label | Minutes | Minutes in middle/high | RPE | Load |
|---|---|---|---|---|---|---|
| Tue | Easy run + strides | Low | 40 | 0 | 3 | 120 |
| Thu | 15 min warm-up, 3 × 8 min threshold with 2 min jogs, 10 min cooldown | Middle | 53 | 24 | 7 | 371 |
| Sat | Long run, easy | Low | 75 | 0 | 4 | 300 |
| Sun | Easy recovery run | Low | 35 | 0 | 2 | 70 |
| Total | 3 low, 1 middle | 203 | 24 | 861 |
By time, 24 of 203 minutes is about 12% middle, so roughly 88 / 12 / 0. That’s within the build-phase target, though a few short fast reps would bring the high share up toward 5%. Your watch will probably show nearer 18–20 minutes above zone 2, because of heart rate lag in the first minute of each rep.
By session, it’s one hard day in four runs, well spaced, so recovery is under control. The load comes to 861, and next week’s total should sit close to this, or a little above, rather than jumping.
Signs your distribution is drifting
Check your numbers over a rolling four weeks, not week by week. Look for:
- easy runs creeping up: any easy run with time in zone 3, or easy-run RPE scores rising to 5 or more
- hard sessions falling short, where you can’t reach target pace or effort in your reps, or you cut sessions short (this usually means you’re arriving tired, and the cause is usually the easy days)
- more hard days than planned; group runs, races and “just seeing how it feels” all count, so label them for what they were
- a rising resting heart rate or poor sleep alongside any of the above, which is a cue to back off for a few days (sleep and running performance covers why this matters)
The fix is almost always the same: make the easy runs easier. Cutting a hard session is a last resort.
Planning your next training block
Easy runs are easy so your hard runs can be hard, so plan the balance between them and not just the individual sessions. Count in three buckets: low (zones 1–2), middle (zones 3–4) and high (zone 5). Easy means zones 1 and 2 only; in zone 3 or above, you’re going too hard.
Keep around 80% low all year. Split the other 20% pyramidally while building (more middle) and move toward polarized (more high) as you sharpen, if your race distance calls for it. Your race distance decides the race-specific phase: 5K training ends up polarized, while marathon training stays pyramidal because the marathon is run in the middle bucket.
To stay on track, label every session to control your hard days, check time in zone to keep easy runs easy, and log session RPE to track load. If the numbers drift, make the easy runs easier first.
This article is general information, not medical or coaching advice.
References
- Silva Oliveira P, Boppre G, Fonseca H. Comparison of polarized versus other types of endurance training intensity distribution on athletes’ endurance performance: a systematic review with meta-analysis. Sports Medicine, 2024. ncbi.nlm.nih.gov
- Nøst HL, Aune MA, van den Tillaar R. The effect of polarized training intensity distribution on maximal oxygen uptake and work economy among endurance athletes: a systematic review. Sports, 2024. pmc.ncbi.nlm.nih.gov
- Stöggl T, Sperlich B. Polarized training has greater impact on key endurance variables than threshold, high intensity, or high volume training. Frontiers in Physiology, 2014. doi.org
- Casado A, González-Mohíno F, González-Ravé JM, Foster C. Training periodization, methods, intensity distribution, and volume in highly trained and elite distance runners: a systematic review. International Journal of Sports Physiology and Performance, 2022. doi.org
- Filipas L, Bonato M, Gallo G, Codella R. Effects of 16 weeks of pyramidal and polarized training intensity distributions in well-trained endurance runners. Scandinavian Journal of Medicine & Science in Sports, 2022. doi.org
- Seiler KS, Kjerland GØ. Quantifying training intensity distribution in elite endurance athletes: is there evidence for an “optimal” distribution? Scandinavian Journal of Medicine & Science in Sports, 2006. doi.org
- Sylta Ø, Tønnessen E, Seiler S. From heart-rate data to training quantification: a comparison of 3 methods of training-intensity analysis. International Journal of Sports Physiology and Performance, 2014. doi.org
- Foster C et al. A new approach to monitoring exercise training. Journal of Strength and Conditioning Research, 2001. doi.org
