5K Base Phase: The Zone 2 Easy Run

This article is part of our 5K session series. Each article covers one session: how it’s structured, what it does, when to run it, and what to run around it. This one is the most important session in the whole plan, and the easiest to get wrong: the zone 2 easy run.

The session at a glance

Where it fits: the four phases of 5K training

We split 5K training into four phases. Each has a different job:¹

• Base (6–8 weeks): build your aerobic engine and get your legs used to regular running. Almost everything is easy.
• Build (4–6 weeks): add harder sessions, like threshold intervals, on top of the base.
• Peak (2–3 weeks): sharpen with work at or near 5K race pace.
• Taper (7–10 days): cut the volume, keep a little speed, and arrive at the start line fresh.

The zone 2 easy run is the core of the base phase, and it doesn’t disappear afterward. It stays in your week all the way through, as the run that holds everything else together.

Purpose, goal and target

Purpose: to build your aerobic base, your body’s ability to produce energy using oxygen, steadily, for a long time. A 5K is a mostly aerobic race, so this is the engine you’ll be racing on.

Goal: to cover 4.3 miles (7km) with your heart rate in zones 1 and 2 the whole way, finishing feeling like you could have kept going.

Target: zones 1 and 2 only, for 100% of the run, with not a single minute in zone 3. If you’re not sure how to set your zones, how to build an aerobic base walks through it step by step. The effort is 3 or 4 out of 10, very easy: you can hold a full conversation, and it should feel almost too slow. There is no pace target. Pace is whatever keeps your heart rate in zone, and on a hot or tired day it will be slower, which is fine.

How to run it

1. Start with 5 minutes of brisk walking. This lets your heart rate rise gradually instead of spiking when you start running.
2. Ease into a slow jog, keeping the first mile deliberately slower than feels necessary. Heart rate lags behind effort, so a pace that feels fine in the first few minutes often puts you in zone 3 by the second mile.
3. Settle into zone 2 with a short, light stride. Breathe comfortably; most people can breathe through their nose at this effort.
4. Walk when your heart rate goes too high. If you go over the top of zone 2, walk until it drops back toward the middle of the zone, then jog again. Set the top of zone 2 as a heart rate alert on your watch so you don’t have to keep checking.
5. Finish with a few minutes of walking, and no sprint finish.

The walking rule is the part people resist, but it’s what makes the session work. Walking to stay in zone is part of the run, not a failure of it. As your base improves, the walks get shorter and less frequent, and eventually disappear.

What it does to your body

Easy running at this intensity drives the adaptations that make you a better distance runner. Sustained low-intensity work increases the number of mitochondria (the parts of muscle cells that turn fuel into energy) and grows the network of small blood vessels that deliver oxygen to your muscles.² Your heart pumps more blood with each beat, so over time it needs fewer beats to hold the same pace, and your muscles get better at using fat as fuel, saving carbohydrate for harder efforts.

Your legs toughen up too, though slowly. Tendons, bones and joints adapt to the repeated impact of running, and easy running builds up the time on your feet without the extra load of faster running.³

This is also why most of your running should be easy. When researchers looked at how successful endurance athletes train, they found roughly 80% of training done at low intensity.⁴ Among competitive runners, the group that did more of their training at low intensity improved more than the group that did more moderate work.⁵

Timing

Run it once or twice a week in the base phase. It isn’t your longest run of the week, but your long run may well sit in the same easy category, run at the same zones and effort over a greater distance.

Pick a day when you have time and aren’t rushed; many runners use a weekend morning. Any time of day works as long as you’ll do it, though in summer an early morning or evening run keeps you out of the heat, which pushes heart rate up.⁶ For fueling, leave 2–3 hours after a full meal. If you need something closer to the run, keep it small, like a banana.

If 4.3 miles (7km) is too far at first, run it by time instead, starting at 30 minutes and adding 5 minutes every week or two. Once 4.3 miles (7km) in zone 2 is comfortable, let your pace improve at the same heart rate rather than adding speed.

Matching sessions

In the base phase, the zone 2 easy run sits alongside two other sessions in this series: the 5K easy run with strides and the steady tempo run. A typical base week with three or four runs looks like this:

Beginners can drop the steady tempo run for the first few weeks and replace it with another easy run. In the build phase, the zone 2 run stays in place and the threshold intervals session takes over as the harder run of the week.

If you want to add more, low-impact cross-training such as cycling at the same easy heart rate builds aerobic fitness without adding impact. A short core session, like the ones in core workouts for runners, also fits well on the same day or the day after.

Back-to-back: the day before and the day after

Because it’s easy, the zone 2 run is the most flexible session in the week.

The day before: rest or an easy day is best, and a short easy run or cross-training is fine. Avoid a hard session the day before. If your legs are tired, you’ll struggle to stay in zone 2, and you’ll end up walking far more than usual.

The day after: rest, or light movement like walking or gentle cycling, is best. There’s a good case for active recovery over complete rest on this day. Your next quality session, such as the steady tempo run, is also fine. Research on heart rate recovery found that the body recovers quickly from exercise below the first ventilatory threshold, roughly the top of zone 2, and more slowly from anything harder.⁷ An easy run done properly shouldn’t leave you too tired for a harder session the next day. If you’re sore or your resting heart rate is higher than usual the next morning, the run wasn’t as easy as it should have been.

Common mistakes

• Running it too fast. This is by far the most common one. A run at 5 or 6 out of 10 feels productive, but it’s too hard to recover from quickly and too easy to count as a proper hard session. It’s near the top of the list of common beginner running mistakes.
• Refusing to walk. Pushing on with your heart rate in zone 3 turns this into a different session.
• Chasing a pace. Set your watch to show heart rate as the main field, not pace.
• Trusting a wrist sensor. Wrist-based heart rate readings can be unreliable while running, and a chest strap is far more accurate for zone training.
• Running hills. A steep climb can push you out of zone 2 in under a minute, so choose a flat route.

Before your next easy run

The zone 2 easy run is the foundation of 5K training: 4.3 miles (7km), zones 1 and 2 only, at 3 or 4 out of 10. It builds the aerobic engine a 5K runs on, and it stays in your week through every phase. Pick a flat route, start gradually, ignore your pace, and walk whenever your heart rate goes too high; that walking is part of the session. In the base phase, pair it with the easy run with strides and the steady tempo run, and keep hard sessions away from the day before.

This article is general information, not medical or coaching advice. If you feel chest pain, dizziness or unusual breathlessness while running, stop and talk to a doctor.

References

1. Issurin VB. New horizons for the methodology and physiology of training periodization. Sports Medicine, 2010. doi.org
2. Holloszy JO, Coyle EF. Adaptations of skeletal muscle to endurance exercise and their metabolic consequences. Journal of Applied Physiology, 1984. journals.physiology.org
3. Gabbett TJ. The training-injury prevention paradox: should athletes be training smarter and harder? British Journal of Sports Medicine, 2016. doi.org
4. Seiler S. What is best practice for training intensity and duration distribution in endurance athletes? International Journal of Sports Physiology and Performance, 2010. doi.org
5. Esteve-Lanao J et al. Impact of training intensity distribution on performance in endurance athletes. Journal of Strength and Conditioning Research, 2007. doi.org
6. Coyle EF, González-Alonso J. Cardiovascular drift during prolonged exercise: new perspectives. Exercise and Sport Sciences Reviews, 2001. doi.org
7. Seiler S, Haugen O, Kuffel E. Autonomic recovery after exercise in trained athletes: intensity and duration effects. Medicine & Science in Sports & Exercise, 2007. doi.org