Interval Training for Runners — Workouts That Build Speed
Interval training is the fastest way to get faster — short bursts of hard running, separated by recoveries, repeated until the body learns to hold a pace it could not hold continuously. It is also the session most runners do wrong: too fast, too long, too often, with recoveries too short, until the workout becomes a test of willpower rather than a training stimulus.
This guide explains how intervals actually work, the four main types (400s, 800s, fartlek and hills), the paces and recoveries for each, and how to fit one interval session a week into a plan without wrecking the easy days around it. Intervals are a precision tool, and precision is what makes them effective.
Use the calculator
Start with your own numbers: try our pace calculator, running time predictor to work out your figures before applying anything on this page. The worked examples below use round numbers so you can follow the maths; the calculators use your real weight, height, age and goal.
Related reading: Tempo Runs Explained — How to Run Them and Why They Work, How to Run a Faster 5K — Training, Pacing and Race Day, How to Improve Your Running Pace — 7 Proven Methods. You can also browse the full guides section for the rest of the series.
The key points
Intervals work by accumulating time at high intensity. A single 400 m repetition is not the stimulus; the accumulation of 6–8 repetitions, each at goal pace with a short recovery, is. The body adapts to the total time spent near maximum oxygen uptake, and intervals are the way to accumulate that time without the exhaustion of a continuous hard effort. The recovery is part of the session, not a break from it — it is what makes the next repetition possible.
400 m repetitions are the classic 5K session. Six to eight × 400 m at 5K goal pace (or 5–10 seconds per kilometre faster) with 90 seconds of jogging recovery teaches the body to hold race pace when tired. The pace should feel hard by the last two repetitions but controlled — if you cannot finish the last repetition at the target pace, the pace was too fast or the recoveries too short.
800 m repetitions suit 10K and half-marathon training. Four to six × 800 m at 10K goal pace with 2 minutes of recovery builds the ability to hold a fast pace for longer — which is exactly what the second half of a longer race demands. The 800s are run slightly slower than the 400s but for twice as long, and the total time at pace is similar. Choose the distance that matches your race: 400s for 5K, 800s for 10K and beyond.
Fartlek is intervals without a track. Swedish for "speed play", fartlek means alternating fast and easy running by feel — 2 minutes fast, 2 minutes easy, for 20–30 minutes — on any route, without measuring distances. It is the most flexible interval session, the easiest to fit into a normal run, and the best introduction to speed work for runners who find the track intimidating. The paces are approximate, and that is the point: fartlek trains effort, not numbers.
Hill repetitions build strength and form. Short, steep hills (30–60 seconds up, jog back down) build leg strength, improve running economy, and force a good form — high knees, quick cadence, powerful arms — without the impact of flat speed work. One hill session every two weeks is enough for most runners, and it is the lowest-injury-risk form of speed work, because the hill limits the speed and the impact.
One interval session a week is enough. Intervals are the most fatiguing session in the week, and they need 48 hours of easy running or rest around them — never run intervals the day after a tempo run, and never run a tempo the day after intervals. Two interval sessions a week is possible for experienced runners, but the second one usually costs more in fatigue than it gains in fitness. The intervals are the spice, not the main dish: the easy runs are the meal.
At a glance
| Session | Repetitions | Pace | Recovery | Total time |
|---|---|---|---|---|
| 400 m (5K) | 6–8 × 400 m | 5K goal pace | 90 s jog | 25–30 min |
| 800 m (10K) | 4–6 × 800 m | 10K goal pace | 2 min jog | 30–35 min |
| 1,000 m (10K) | 4–5 × 1,000 m | 10K goal pace | 90 s jog | 30–35 min |
| Fartlek | 2 min fast / 2 min easy × 6 | By feel, hard | Equal easy | 25–30 min |
| Hills | 8–10 × 45 s uphill | Hard effort | Jog back down | 25–30 min |
| 200 m (speed) | 8–10 × 200 m | Faster than 5K pace | 200 m jog | 20–25 min |
Always warm up for 10–15 minutes easy before intervals, and cool down for 10 minutes after. The warm-up is not optional — cold intervals are how injuries happen.
The formula behind it
Interval paces are calculated from goal race pace: pace per kilometre = target time ÷ distance. A 25-minute 5K needs 5:00 per kilometre, so a 400 m repetition at goal pace takes 5:00 × 0.4 = 2:00, and an 800 m repetition takes 4:00. A 50-minute 10K needs 5:00 per kilometre too, so the 800 m pace is the same — which is why the two distances share a pace, and why the difference is the number of repetitions, not the speed.
The Riegel formula (Riegel, 1981) predicts the interval pace from another distance: time₂ = time₁ × (distance₂ ÷ distance₁)^1.06. A 30-minute 5K predicts a 10K of 30 × 2^1.06 ≈ 62.5 minutes, or 6:15 per kilometre — so the 800 m repetitions for that runner are 6:15 × 0.8 = 5:00 each. The calorie maths is the Compendium (Ainsworth et al., 2011): hard running at 7.5 mph (11.5 MET) burns 11.5 × weight (kg) per hour, so a 30-minute interval session at 75 kg burns about 430 kcal — but the session's value is the speed adaptation, not the calories.
Your plan, step by step
- Warm up for 10–15 minutes easy, with a few short strides to prepare the legs.
- Run the repetitions at goal pace — not faster — with the recoveries in the table.
- Stop the session if you cannot hold the pace for two consecutive repetitions; the pace was too fast.
- Cool down for 10 minutes easy, and keep the next day easy or rest.
- Do one interval session a week, and progress by adding a repetition — not by running faster — every 2–3 weeks.
How to use this guide
Use intervals as the weekly speed test: if the repetitions feel easier than last month at the same pace, you are fitter, and the goal pace should be adjusted. If they feel harder, you are tired — shorten the session or skip it, rather than forcing it. Intervals are honest in a way that easy runs are not: they tell you, every week, where your speed actually is.
Practise the discipline of the pace. The most common interval error is running the first repetition too fast — excited, fresh, and 10 seconds ahead of target — and then fading through the rest of the session. Run the first repetition at the target pace, not faster, and let the session build. The runners who improve most from intervals are not the ones who run the fastest first repetition; they are the ones who run the last repetition at the same pace as the first.
What this guide does NOT tell you
Interval paces are population averages: your interval pace, recovery needs and the number of repetitions you can handle depend on your fitness, age and training history, and the Riegel prediction is an estimate, not a guarantee. Intervals are the highest-injury-risk session in running — the combination of speed, fatigue and impact loads the muscles, tendons and joints more than any other session — and they should be built gradually, not jumped into from a low base.
This guide is general information, not medical advice. Anyone with a heart condition, joint problem or medical condition affecting exercise should check with a clinician before starting interval training, and the warm-up is not optional: cold intervals are a well-known cause of muscle strains. Sharp pain, chest pain or severe breathlessness during intervals are signals to stop immediately and get checked.
Frequently asked questions
How often should I do interval training?
Once a week is enough for most runners, with easy days or rest around it — never run intervals the day after a tempo run, and never run a tempo the day after intervals. Two interval sessions a week is possible for experienced runners, but the second one usually costs more in fatigue than it gains in fitness. The intervals are the smallest part of the recipe by volume; the easy runs are the meal, and the intervals are the spice.
What pace should I run my intervals at?
At your goal race pace, or 5–10 seconds per kilometre faster for short repetitions (200–400 m). For a 25-minute 5K goal (5:00 per km), run 400 m repetitions in about 2:00 each. Running the intervals much faster than goal pace is counterproductive — it teaches a pace you cannot hold in the race and leaves you too tired for the tempo run. The last repetition should be hard but at the same pace as the first.
Are intervals better than tempo runs?
They do different jobs, and a good week has one of each: intervals build top speed and the ability to hold a fast pace when tired, while tempo runs build the lactate threshold — the speed at which fatigue accumulates. Tempo runs are the more valuable session for most runners, because the threshold is the main limiter; intervals add the speed that sits on top of it. Skipping either one leaves a gap the other cannot fill.
What is fartlek training?
Swedish for "speed play": alternating fast and easy running by feel, without measuring distances — for example, 2 minutes fast and 2 minutes easy, repeated for 20–30 minutes, on any route. It is the most flexible interval session, the easiest to fit into a normal run, and the best introduction to speed work. The paces are approximate, and that is the point: fartlek trains effort and pace judgement, not numbers on a watch.
Do I need a track for interval training?
No — a flat, measured stretch of path, a park loop, or a treadmill all work, and fartlek needs no measurement at all. A track is convenient because the distances are marked and the surface is flat, but it is not necessary: a GPS watch measures 400 m on any route, and the repetitions work the same. What matters is the pace and the recovery, not the venue — and the warm-up and cool-down matter more than either.
Estimates only, not medical advice. Minimum safe intakes: 1,200 kcal/day women, 1,500 kcal/day men — consult a healthcare professional before a very-low-calorie diet.