5K Build Phase: Threshold Intervals

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 first proper hard session in the plan, and it marks the move from the base phase into the build phase.

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.

After 6–8 weeks of mostly easy running, your aerobic base is in place. The build phase adds harder work on top of it, and this session is where that starts. It replaces the steady tempo run as the key session of the week.

Purpose, goal and target

Purpose: to raise your lactate threshold, the fastest pace you can sustain before fatigue builds rapidly, and to introduce your body to running at a hard effort in a controlled way.

Goal: three evenly paced 0.3-mile (500m) reps at threshold effort, with the last rep as quick as the first, and full control throughout.

Target:

• Threshold effort is 8 out of 10: comfortably hard, with enough breath for a few words but not a conversation.
• Heart rate should rise into zone 4 by the end of each rep. Because the reps are short, your heart rate will lag behind and may only reach zone 4 near the end, so judge the first rep by effort, not by your watch.
• Recoveries are an easy jog, bringing you back down into zone 2 or low zone 3 before the next rep starts.
• If you know your 5K time, threshold pace is a little slower than 5K race pace. If you don’t, use effort alone.

How to run it

Warm-up: 1.2 miles (2km)

1. Start with a few minutes of brisk walking.
2. Jog the 1.2 miles (2km) easily in zone 2, building slightly over the last few minutes.
3. Optional: add 2–3 short strides at the end to get your legs ready for faster running. A proper warm-up raises muscle temperature and improves performance in the hard efforts that follow.²

Main set: 3 × (0.3 mile / 500m threshold + 0.3 mile / 500m easy jog)

4. Rep 1: build into threshold effort over the first 50–100 yards (50–100m), then hold it. On a standard 400m track, 0.3 mile (500m) is one and a quarter laps.
5. Recovery: slow right down to an easy jog for 0.3 mile (500m). Walk briefly if you need to, but keep moving.
6. Reps 2 and 3: same effort, same pace. If rep 3 is much slower than rep 1, you started too fast.

Cooldown: 0.6 mile (1km)

7. Jog easily, letting your heart rate come back down, then walk for a few minutes.

What it does to your body

Training at or near threshold pushes your lactate threshold up, so you can run faster before fatigue sets in.³ In a 5K, that means you can hold a quicker pace for longer. Your body also gets better at buffering and clearing the byproducts of hard efforts, and your mind gets used to the feeling of running uncomfortably.

Repeated efforts at high intensity push your heart and lungs harder than continuous easy running can, which stretches your aerobic capacity.⁴ Breaking threshold running into short reps with recoveries keeps the dose manageable: you spend good-quality time at a hard effort without the fatigue of one long continuous effort, which makes it a good first interval session.

Hard sessions work best on top of a large amount of easy running. Studies comparing training approaches have found the biggest improvements when a small amount of hard work is combined with a lot of low-intensity running, rather than lots of moderate work.^{5, 6} That’s why the zone 2 runs stay in your week.

Timing

This is your one hard session, so run it once a week, early or mid-week when you’re fresh, with an easy day or rest either side. Go into it rested and fueled: leave 2–3 hours after a full meal, though a small, easily digested snack an hour before is fine.

A track is ideal because the distances are exact, but a flat, measured path or a GPS watch works too.

Through the build phase, add a rep every week or two, up to 5 × 0.3 mile (500m), or lengthen the reps to 0.5–0.6 mile (800m–1km) with the same recovery distance. Only increase once three reps feel controlled. In the peak phase, this session gives way to work at 5K race pace, with threshold intervals appearing less often.

Matching sessions

In the build phase, the threshold intervals take over from the steady tempo run as the hard session. A typical week:

Runners with more experience can add a fourth run, such as another zone 2 easy run on Sunday. Keep building total volume slowly; adding hard sessions and more mileage at the same time is a common route to injury. See building weekly mileage safely.

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

The day before: rest, or an easy run with strides, is best, and strides the day before can leave your legs feeling sharp. Avoid another hard or long session. Intervals on tired legs mean slower reps, poor form and a higher risk of injury.

The day after: rest or active recovery is best. A short zone 2 easy run is fine if you keep it easy, walking if your heart rate goes too high. Avoid the steady tempo run or any other session in zone 3 or above. Your body takes longer to recover from efforts above easy running,⁷ so the day after a hard session should always be easy or off. Eat a meal with carbohydrates and protein within a couple of hours, and prioritize sleep; see sleep and running performance.

Common mistakes

• Running the first rep too fast. Short reps feel easy at first, and if you can’t match your first rep’s pace on the third, it was too fast.
• Turning recoveries into rest stops. Keep jogging; the recovery is part of the session.
• Treating it as a sprint session. Threshold is 8 out of 10, not all-out. Save the faster work for the peak phase.
• Skipping the warm-up. Hard running on cold legs feels worse and raises the risk of strains.
• Adding it too early. Without a solid base, intervals are more likely to leave you tired or injured than faster. If you’re still working through Couch to 5K, finish that first.

Your first threshold session

The session is a 1.2-mile (2km) warm-up, 3 × 0.3 mile (500m) at threshold with 0.3-mile (500m) easy jogs, and a 0.6-mile (1km) cooldown: 3.7 miles (6km) in total. Threshold is 8 out of 10, comfortably hard, with your heart rate rising into zone 4. Running there raises your lactate threshold, the pace you can hold before fatigue builds rapidly, which is central to 5K performance. Pace the reps evenly, keep jogging through the recoveries, and never skip the warm-up. Run it once a week with rest or easy running on either side, and keep the zone 2 runs in your week.

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. Bishop D. Warm up II: performance changes following active warm up and how to structure the warm up. Sports Medicine, 2003. doi.org
3. Faude O, Kindermann W, Meyer T. Lactate threshold concepts: how valid are they? Sports Medicine, 2009. doi.org
4. Buchheit M, Laursen PB. High-intensity interval training, solutions to the programming puzzle. Part I: cardiopulmonary emphasis. Sports Medicine, 2013. doi.org
5. 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
6. 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
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