What Is Interval Training? A Plain-English Guide for Runners

Plenty of new runners do interval training without realising. A physio’s instruction to walk two minutes, jog one, and repeat six times around the local park is interval training. So is the couch to 5K app barking at you to run for 60 seconds and walk for 90. Three weeks of either and resting heart rate drops, the hill on the commute stops hurting, and progress is measurable. The label “intervals” sounds elite. The basic principle is anything but.

Interval training is not an elite-only protocol. It is one of the oldest, simplest training ideas in endurance sport, and you have probably already done it without realising. This guide explains what it is, why it works, and what the different types look like.

Interval Training, Defined

Interval training is any structured workout that alternates harder efforts with periods of easier effort or rest. That is the whole definition. The hard bit can be a 30 second sprint or a four minute climb at threshold pace. The easy bit can be a slow jog, a walk, or standing still. What makes it interval training rather than a steady run is the deliberate switching between the two.

Steady-state running, by contrast, holds one effort the whole way through. Useful, but limited. Intervals push you into harder zones a normal run would never visit.

The format you choose – the length of the hard rep, the length of the recovery, and the intensity of each – shapes the adaptation. Short and very fast trains pure speed. Longer and moderately hard trains your threshold. Hilly repeats train strength and form. All interval training; just pointing your fitness in different directions.

Why Intervals Work: The Physiology in Everyday Terms

Running fast is uncomfortable. That discomfort is the whole point. When you push harder than your normal cruising pace, three things start happening inside your body that a steady jog cannot trigger to the same extent.

VO2 max – your aerobic ceiling

VO2 max is the most oxygen your body can use during hard exercise. Think of it as the size of your aerobic engine. Repeated efforts at or near your maximum oxygen uptake stretch that ceiling upward. The classic work by Veronique Billat at Paris-Sud University on intervals at “vVO2max” (the pace that elicits your VO2 max) showed that this kind of training produced larger aerobic gains than the same volume of moderate continuous running.

Anaerobic threshold – the pace where everything starts to hurt

There is a pace, somewhere between your easy jog and your all-out sprint, where lactate starts building up in your blood faster than your body can clear it. That pace is your anaerobic threshold. Below it, you can keep going for ages. Above it, the clock starts ticking. Interval work at and around this pace teaches your body to clear lactate more efficiently, which pushes the threshold higher. The higher it sits, the faster you can run before everything starts to burn.

Lactate clearance – the part most people misunderstand

Lactate is not the villain it was once thought to be. It is a fuel your muscles produce and reuse. Under heavy load, production outpaces reuse and the byproducts make your legs feel heavy. Stephen Seiler’s research on polarised training, and decades of work in Jack Daniels’ Running Formula, point to the same conclusion: regular doses of harder intervals improve your body’s ability to recycle lactate quickly, one of the clearest markers of endurance fitness.

You do not need to remember any of these terms mid-session. You only need to know that short, structured doses of harder effort produce changes easy running alone cannot.

The Main Types of Intervals

“Intervals” is an umbrella term. Underneath it sit several different formats, each with a slightly different purpose. You do not need to use all of them. You just need to know they exist so you can recognise what a plan is asking you to do.

Short sprints

Reps of 100 m to 200 m at near-maximum effort, with long recoveries (60 to 90 seconds of walking or standing). The point is pure speed and neuromuscular coordination. Quality, not volume.

Track repeats (400 m, 800 m, 1 km)

The classic interval session. Reps at 5 km to 10 km race pace, with jog recoveries between. These are what most people picture when they hear “intervals”. They build VO2 max and the ability to hold pace under fatigue.

Fartlek

Swedish for “speed play”. Unstructured: pick a tree, a lamppost, or a song chorus, run hard until you reach it, then jog until you are ready to go again. The most beginner-friendly way to introduce harder running because the structure is whatever you decide on the day.

Hill intervals

Short, steep climbs (30 seconds to 2 minutes) at hard effort, with an easy jog or walk back down as recovery. Hills give you a built-in speed limit that protects your form, while delivering the same cardiovascular load as flat intervals. They also build leg strength in a way flat running cannot.

Tempo and threshold intervals

Longer reps (3 to 10 minutes) at “comfortably hard” pace – the kind where you could just about manage a one-word answer. Shorter recoveries (1 to 2 minutes of jogging). The workhorse session for half marathon and marathon runners because they sit right at the edge of your anaerobic threshold.

Walk-run intervals

The beginner format described at the top of this article. Alternating a set period of walking with a set period of running, repeated for the duration of the session. Programmes like Couch to 5 km are built entirely on this principle. Interval training stripped to its absolute basics.

Who Interval Training Is For (and Who It Is Not)

Almost any runner with a base of consistent training can benefit from intervals. That said, there is one group that should pause before adding fast running to the schedule: complete beginners.

If you have been running for less than 8 to 12 weeks, your tendons, ligaments and bones are still adapting to repeated impact. Adding hard reps on top of that can tip you into the injury zone before you have built the structural resilience to handle it. The exception is the walk-run format above, which is gentle enough to count as base-building rather than speed work.

For everyone else – the runner stuck at the same 10 km time, the new parent fitting workouts into 30 minute slots, the experienced marathoner who needs a sharpener – intervals are one of the highest-return additions you can make to a week. One session every seven days is enough to produce measurable change.

If you want a schedule that builds intervals in at the right pace for your fitness, the RunReps Running Plan Generator builds a personalised programme around your goal race, your current pace, and your available training days.

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How Intervals Compare to Steady-State Running

Steady running builds aerobic base, capillary density and fat-burning efficiency. It is the bedrock of every endurance plan. But on its own, steady running plateaus – the body adapts to a single stimulus and then stops adapting.

Intervals are the second ingredient. They push you above your comfort zone for short, controlled doses, forcing further adaptation. The best programmes use both: roughly 80 percent of weekly running at easy, conversational pace, the remaining 20 percent split between intervals, tempo work and race-pace efforts. That ratio comes out of Seiler’s polarised training research and is now the default model for most endurance coaching. Without intervals, the easy running has nothing to push against. Without the easy running, the intervals burn you out.

What a Simple Interval Session Looks Like

To make this concrete, here is what an early interval session might look like for someone who has been running consistently for three months and wants to get faster.

• Warm-up: 10 minutes easy jog, plus a few strides (short pickups of 20 to 30 seconds at fast but controlled effort).
• Main set: 6 x 400 m at a hard but sustainable effort (roughly your current 5 km race pace), with 90 seconds of slow jogging between each rep.
• Cool-down: 10 minutes easy jog, then a few minutes of walking.

Total session time: about 35 to 45 minutes. Total hard running: 2.4 km. That small amount of hard work, repeated weekly, is enough to start shifting your VO2 max and threshold pace upward within a few weeks.

If you want a session matched to your exact pace, the RunReps Intervals tool builds custom workouts in seconds. Pair it with the pace to heart rate zone calculator if you want to use heart rate as a sanity check on whether you are running each rep at the right effort.

Where to Go From Here

You now know what interval training is, why it works, and what the main formats look like. The next step is actually running one. For session-by-session structures, pacing rules, recovery guidance and the four interval workouts most useful for recreational runners, read interval training for runners: build speed without burning out. That guide picks up where this one ends – it shows you how to do the workouts, not just what they are.

Common Questions About Interval Training

What is interval training in simple terms?

Interval training is any workout that alternates short bursts of harder effort with periods of easier effort or rest. The hard bit pushes your body into a higher gear; the easy bit lets you recover enough to do it again. It works for running, cycling, swimming, rowing, and almost any rhythmic exercise.

Is interval training the same as HIIT?

HIIT (high-intensity interval training) is one type of interval training, usually involving very short, very hard efforts (20 to 60 seconds) with brief recoveries. All HIIT is interval training, but not all interval training is HIIT. A 4 x 1 km session at 10 km race pace, for example, is interval training but not HIIT – the intensity is sustained rather than maximal.

How often should beginners do intervals?

Once a week is plenty when you are starting out, and only after you have built a base of 8 to 12 weeks of consistent easy running. Two interval sessions a week is the upper limit for recreational runners and should only come in after several months of single-session weeks.

Do I need a track to do intervals?

No. A track makes measuring distance easier, but you can run intervals on a park path, a quiet road, a treadmill, or a hill. Time-based intervals (run hard for 2 minutes, jog for 1) remove the need to measure distance at all and work anywhere.

This article is for informational purposes only and does not constitute medical advice. If you are new to running, returning after injury, or have any underlying health conditions, consult a qualified healthcare professional before starting an interval programme.