Interval training, alternating hard effort with recovery, is the most time-efficient way to improve running speed and VO2 max. In a session of 35–45 minutes (including warm-up and cool-down), you accumulate 15–25 minutes of quality effort that would be impossible to sustain continuously. The recovery periods allow each hard segment to be run at genuinely high quality rather than a compromised pace driven by accumulated fatigue.

Research confirms that runners who incorporate intervals improve more quickly than those doing equivalent mileage at steady effort — and at a comparable or lower injury risk when built progressively.

Use our Pace Calculator to identify your interval and tempo paces based on recent race performance.

Our training plan hub integrates interval sessions into structured builds toward 5K, 10K, half marathon, and marathon goals.

What Interval Training Actually Does

When you run at high intensity, the oxygen demand exceeds what your aerobic system can supply immediately. The body responds by recruiting more fast-twitch muscle fibres, increasing stroke volume (blood pumped per heartbeat), improving the capacity to buffer and clear metabolic byproducts, and, over time through repeated stimulus, increasing mitochondrial density and VO2 max.

The recovery interval matters as much as the hard segment. Active recovery (jogging between intervals rather than standing still) maintains blood flow that supports metabolic clearance, keeping muscles primed for the next high-quality rep. This is why interval training produces more total quality work per session than a single continuous hard effort of equivalent duration.

What interval training develops:

• VO2 max (aerobic ceiling) — through VO2 max-specific intervals
• Lactate threshold (the pace you can sustain aerobically) — through tempo intervals
• Running economy (oxygen cost at any given pace) — through all types of fast running
• Speed and neuromuscular coordination — through strides and sprint intervals

For the full physiology of how VO2 max adapts to interval training, see our VO2 max guide.

The Four Main Types of Interval Training for Runners

Type 1: Fartlek (Speed Play)

Fartlek — Swedish for “speed play” — is unstructured interval training within a continuous run. You accelerate when you feel like it (or to a landmark: the next lamp post, the crest of a hill, a specific distance), then ease back to an easy pace until you’re ready to go again. No stopwatch, no set reps.

When to use it: Ideal for beginners introducing faster running for the first time, and for experienced runners returning from a break or in early base-building phases. Also useful for mentally breaking up a long easy run.

Example session:

• 10 min easy warm-up
• 20 min run with 6–8 surges of 30–60 seconds at 5K effort, returning to easy pace between each
• 5 min easy cool-down

The value: Fartlek teaches the body to tolerate faster paces without the psychological and physiological pressure of a structured interval session.

Type 2: VO2 Max Intervals

Short-to-medium repetitions (3–5 minutes per rep) at approximately your 3K–5K race effort — roughly 90–95% of maximum heart rate. These directly stimulate the cardiac and muscular adaptations that increase your VO2 max ceiling.

When to use it: The core quality session for runners targeting performance improvement at any distance. Introduced after 4–6 weeks of aerobic base work; typically done 1–2 times per week during a build phase.

Pacing: Approximately your current 3K–5K race pace. The effort should be high but controlled — you should be able to complete the final rep at the same pace as the first. If you’re slowing significantly by rep 3, the session is too intense or recovery is insufficient.

Heart rate: 90–95% of your maximum HR during the work interval. Recovery jog until HR drops to approximately 65–70% of max before the next rep.

Example sessions:

Classic VO2 max:

• Warm-up: 15 min easy + 3 × 20-second strides
• Main set: 5 × 4 minutes at 5K effort / 3-minute easy jog recovery
• Cool-down: 10 min easy

Shorter reps, higher intensity:

• Warm-up: 15 min easy + strides
• Main set: 8 × 2 minutes at 3K effort / 2-minute easy jog recovery
• Cool-down: 10 min easy

Longer reps (for half marathon/marathon fitness):

• Warm-up: 15 min easy
• Main set: 3 × 8 minutes at 10K effort / 3-minute easy jog recovery
• Cool-down: 10 min easy

For a comprehensive look at how VO2 max responds to interval training over time, see our VO2 max guide.

Type 3: Threshold / Tempo Intervals

Sustained running at approximately your one-hour race pace — the pace you could maintain for 60 minutes in a race. This corresponds to roughly 80–85% of maximum heart rate. The purpose is to raise your lactate threshold: the fastest pace your body can sustain primarily aerobically.

Two formats:

Continuous tempo run: 20–40 minutes of sustained running at threshold pace. The classic format. Demands good pacing discipline — start slightly under threshold and build.

Tempo intervals (cruise intervals): Multiple reps of 5–15 minutes at threshold pace with 1–3 minutes recovery jog between. Allows more total time at threshold pace than a single continuous run, with slightly lower mental demand.

Pacing: “Comfortably hard” — you can speak short phrases but not hold a conversation. Not a sprint; not an easy run.

Heart rate: 80–85% of HRmax throughout the work interval.

Example sessions:

Continuous tempo:

• Warm-up: 15 min easy
• Main set: 25 minutes at threshold pace (HR at 80–85% max)
• Cool-down: 10 min easy

Cruise intervals:

• Warm-up: 15 min easy
• Main set: 4 × 8 minutes at threshold / 90-second easy jog recovery
• Cool-down: 10 min easy

Distance-based tempo:

• Warm-up: 15 min easy
• Main set: 3 × 2,000m at threshold pace / 400m easy jog recovery
• Cool-down: 10 min easy

Type 4: Strides and Sprint Intervals

Very short, fast accelerations — 80–150 metres at approximately race pace or slightly faster — with full recovery between. Strides are not a hard session; they’re a neuromuscular activation tool typically used at the end of easy runs or before a race.

When to use them: 2–3 × per week at the end of easy runs throughout a training block. Also used in the warm-up before a key race or quality session.

How to run a stride: Accelerate smoothly over 20–30m, hold the faster pace for 60–80m, decelerate smoothly. Full recovery between each (60–90 seconds walk or stand). No sprint-and-stop; the emphasis is on relaxed, fast mechanics.

Benefit: Develops running economy and neuromuscular coordination without significant training stress. Elite runners do strides almost daily.

Judging Your Pace: The RPE Scale for Runners

When you don’t have access to a GPS or heart rate monitor, the Rate of Perceived Exertion (RPE) scale gives you a reliable effort gauge:

RPE	Description	Running application
1–2	Almost no effort	Walking slowly
3–4	Light effort, could sustain indefinitely	Easy recovery run
5–6	Moderate, comfortable	Zone 2 easy run, long run
7	Somewhat hard, aerobic ceiling	Steady state / marathon pace
8	Hard, need to focus	Threshold / tempo pace
9	Very hard, speaking words only	VO2 max intervals
10	Maximum, unsustainable beyond seconds	All-out sprint

For interval sessions: VO2 max intervals should feel like 9 throughout; threshold tempo like 8 sustained; fartlek surges like 7–8.

For the full heart rate zone framework that maps to these effort levels, see our heart rate while running guide.

Pacing Rules for Intervals

Consistency is the primary goal. The final interval of a session should be run at the same pace as the first. If your pace drops significantly across the session, the intervals are too fast, too long, or the recovery is too short.

The reason consistency matters: if you run the first two intervals at near-maximum capacity and the remaining intervals at reduced pace, you’re training your body to sprint-and-recover rather than to sustain fast, efficient running. Even distribution of effort produces better physiological adaptation and better race performance.

Many experienced coaches prefer runners to finish feeling like they could complete one more rep. This is not a sign the session was too easy — it’s a sign the session was correctly calibrated. A session that leaves you destroyed provides a poor training signal and typically requires 3–4 extra recovery days.

Warm-Up and Cool-Down: Non-Negotiable for Interval Sessions

Running hard on cold muscles dramatically increases injury risk. The warm-up for an interval session is longer and more specific than for an easy run.

Interval session warm-up (15–20 minutes):

1. 10–12 minutes easy jogging (genuinely easy — HR under 65% max)
2. Dynamic activation: leg swings, walking lunges, hip circles (2 minutes)
3. 3–4 strides of 80–100m at session pace, with full recovery between each

The strides at the end of the warm-up prepare the neuromuscular system for the faster running that follows — starting a VO2 max interval session cold, without this neurological priming, produces significantly worse first-rep quality.

For a complete pre-run activation sequence, see our dynamic warm-up guide.

Cool-down (10–15 minutes):

• 10 minutes easy jogging (not walking)
• 5–7 minutes of static stretching targeting hip flexors, hamstrings, calves, and quads

The cool-down jog is as important for interval sessions as for any other run — it allows heart rate and blood pressure to reduce gradually and begins the recovery process. For the full post-run cool-down protocol, see our cool-down guide.

How Often Should You Do Interval Training?

Beginning runners (less than 6 months of consistent running): Defer structured intervals. Build an aerobic base for 6–8 weeks of easy running before introducing any intensity. Start with fartlek — one session per week of 20–25 minutes with informal surges — before moving to structured sessions.

Developing runners (6+ months, training toward a goal race): One structured interval session per week (VO2 max or tempo). The remaining sessions are easy base running and one long run. Adding a second quality session requires at least 2 easy days between each.

Experienced runners: Two quality sessions per week maximum during a build phase — typically one VO2 max session and one tempo session. Never on consecutive days. The 80/20 principle applies: 80% of training volume at easy effort, 20% at quality effort.

Between interval sessions:

• Minimum 48 hours of easy running between quality sessions
• For VO2 max intervals specifically, 72 hours is preferable
• If muscle soreness from the previous session hasn’t resolved, delay the next quality session

For recovery strategy between interval sessions, including what to eat, how to sleep, and HRV monitoring — see our recovery tips guide.