Intervals vs Tempo vs Fartlek: A Runner's Guide to Speed Workouts

Walk into any running club and ask three different runners what a "tempo run" is and three different answers will come back. Ask the same question about intervals and the answers will be slightly more consistent, but only slightly. Fartlek is treated by most runners as a vaguely Scandinavian word that means "running fast for a bit and then slow for a bit, I guess."

These are not interchangeable workouts. Intervals, tempo runs, and fartlek each target a specific physiological adaptation. Confusing them — running a "tempo" at interval pace, or "intervals" at tempo pace — produces a workout that does neither well.

This guide defines each clearly, explains the system each one trains, and gives a practical framework for choosing the right session in any given week.

The three energy systems behind the three workouts

To understand why these workouts are distinct, it helps to start with the three energy systems that fuel running:

• Aerobic system. Uses oxygen to convert carbohydrate and fat into energy. Slow to engage, almost unlimited capacity. Dominant fuel source for any effort longer than about 90 seconds.
• Anaerobic glycolytic system. Burns carbohydrate without oxygen. Faster to engage, but produces lactate and hydrogen ions that accumulate as fatigue. Dominant from roughly 10 seconds to 2 minutes of all-out effort.
• Phosphocreatine (ATP-PC) system. Pure power. Used for the first 5–10 seconds of all-out sprinting. Refills in 2–3 minutes of rest.

Distance running performance depends mostly on the aerobic system, but the anaerobic system contributes meaningfully at race intensities — and the two interact through lactate threshold, the pace at which lactate production starts outpacing clearance. Improving threshold improves the pace a runner can sustain aerobically before the anaerobic system starts dumping lactate faster than the body can buffer it.

Different workout types stress these systems in different proportions, producing different adaptations.

Tempo runs: training the threshold

A tempo run is a continuous effort at or near lactate threshold pace. Most runners feel this as "comfortably hard" — uncomfortable, but sustainable, without the breath-holding desperation of true interval work.

Physiologically, tempo runs train the runner's ability to clear lactate as fast as the body produces it. Over weeks of consistent tempo work, the threshold itself shifts upward: the runner can sustain faster paces before lactate accumulation forces a slowdown.

The classic Daniels (Jack Daniels, Daniels' Running Formula) definition: tempo pace is roughly the pace a well-trained runner could sustain in a one-hour race. For most amateur runners, that's roughly current half-marathon to 10K race pace, depending on training level.

Heart rate for a true tempo effort sits in roughly the 84–90% of max HR range. Effort by feel: the runner can speak two or three words at a time but not hold a conversation.

The biggest mistake amateur runners make with tempo runs is running them too hard. A "tempo" run done at interval intensity does not train the threshold system effectively — it just produces fatigue without the threshold-shifting adaptation. If the tempo effort feels like an interval, it's too fast.

For deeper coverage of threshold training, see the dedicated piece on heart rate zone training.

Intervals: training peak aerobic and anaerobic capacity

Intervals are repeated hard efforts at or near VO2 max intensity, separated by structured recovery. The hard efforts are deliberately too intense to sustain continuously — that's the entire point. By breaking the session into bouts of work and rest, the runner accumulates more time at peak aerobic intensity than they could in a continuous run.

Most "intervals" in distance training fall into one of two ranges:

• VO2 max intervals: 3–5 minute reps at 3K–5K race pace. Recovery 2–3 minutes. The classic 5 × 1000m or 4 × 1200m sessions.
• Anaerobic capacity intervals: 30 s to 2 min reps at faster than 5K pace, with short recoveries. Trains lactate tolerance and the ability to buffer hydrogen ion accumulation.

The shorter, faster intervals (30 s to 90 s) sit closer to the anaerobic system. The longer ones (3–5 min) sit closer to the aerobic system. Both produce the high heart rates and high lactate concentrations that drive adaptation, but they pull in different proportions from different physiological wells.

Heart rate during the work intervals climbs into the 92–100% of max HR range by the end of each rep. Effort by feel: the runner cannot speak at all during the work bouts. Recovery should bring heart rate down to roughly 130–140 bpm before the next rep starts.

The standard mistake with intervals is starting them too fast. A workout of 5 × 1000m run with the first 1000 in 3:50 and the last in 4:30 is significantly less effective than one run at a steady 4:05–4:10 throughout. The body adapts to the total time spent at high intensity, not the speed of any single rep. Even pacing produces more total time at the target physiological stress.

Fartlek: training adaptability and feel

Fartlek (Swedish for "speed play") is structured chaos. The runner runs continuously, but within that continuous run, surges of harder effort are mixed with easier running. The surges can be timed (30 s hard / 60 s easy, repeated), distance-based (sprint to the next lamp post, jog to the one after), or purely intuitive (run hard when it feels right, ease off when it doesn't).

Originally developed in the 1930s by Swedish coach Gösta Holmér to train his middle-distance runners on forest trails, fartlek is the workout designed for terrain and conditions that don't lend themselves to precise pacing.

What fartlek trains:

• The ability to surge. Race tactics frequently demand surges — responding to a competitor, breaking away on a hill, accelerating through the back half of the final lap. Fartlek practices these surges in a low-stakes setting.
• Effort regulation by feel. Runners who only ever train to a watch struggle when conditions disrupt pace targets (heat, wind, hills). Fartlek builds the internal sense of "this is 5K effort" or "this is tempo effort" independent of pace.
• Mixed energy system stress. The varied intensities mean every fartlek session pulls from multiple energy systems, producing broader (if less specific) adaptation than a single-intensity workout.

Fartlek has no precise heart rate target because the entire workout cycles between intensities. The defining feature is that the runner is regulating effort by feel, not by watch.

Side by side: what each workout actually trains

Which workout for which runner

Beginners (under 12 months of structured running): Fartlek is the safest entry to faster running. The unstructured format teaches effort regulation without demanding precise pacing. Most beginners benefit from one fartlek session per week for the first 3–6 months, then transitioning to short intervals and tempo as fitness establishes.

Intermediates (1–3 years, comfortable with a weekly long run and easy mileage): A weekly tempo run plus a weekly fartlek or interval session is the most productive split. The tempo work raises the threshold; the second session adds variety (fartlek) or peak intensity (intervals) depending on goal race.

Advanced (3+ years, established mileage, history of race-specific training): All three workout types appear in different phases of a periodised plan. Base phase emphasises tempo and fartlek; build phase introduces intervals; race-specific phase narrows to the energy systems closest to the goal distance.

Choosing by goal race distance

Each race distance pulls disproportionately from a particular energy system, and the speed workout mix should reflect that.

• 5K: Heavy on intervals at 3K–5K pace. Tempo provides the strength substrate. Fartlek for early-season variety.
• 10K: Balanced between intervals (5K–10K pace) and tempo runs. Fartlek for surge practice.
• Half marathon: Heavy on tempo. Intervals shift toward longer reps (3–5 min) at 10K pace. Fartlek for terrain variety.
• Marathon: Tempo dominates. Intervals are infrequent and serve to support, not lead, the training. Long tempo progressions become the marquee workouts. See the marathon training plan piece for more on this.

How to structure a week with speed work

The general framework: speed work goes on days bracketed by easy running, and never stacks back-to-back. A typical mid-build week looks like:

Two quality sessions (Tuesday and Friday), separated by 72 hours, with the long run on Sunday. The total quality work makes up roughly 20% of weekly volume — consistent with the 80/20 research on amateur training. We covered the 80/20 case in why easy runs aren't easy enough.

For amateur runners, three quality sessions in a week is the upper limit. Beyond that, recovery quality drops faster than fitness gains accumulate.

The five most common mistakes

Running tempo at interval pace. The single most common error. A tempo session at interval intensity is just a poorly-paced interval session — the runner pays the full recovery cost without getting the threshold adaptation. If the runner can't carry a half-sentence at the prescribed pace, slow down. (For precise paces by zone, the Daniels VDOT system gives you exact tempo, threshold, and interval paces from a recent race — find your VDOT-based pace zones here.)

Running intervals too cautiously. The opposite mistake. Intervals at tempo intensity fail to drive VO2 max adaptation. The work bouts should leave the runner unable to speak; the last rep should feel marginally completable.

Skipping recovery between reps. Cutting interval recovery from 90 s to 60 s "to make it harder" lowers the quality of subsequent reps. The runner pays in pace what they save in rest. Recovery is part of the workout design.

Adding fartlek as filler. Some plans use "fartlek" as a label for any vaguely-faster run that wasn't planned. A productive fartlek has structure — even if loose — and effort distinct from easy running. Sprinkling 30-second pickups into an easy run is not a fartlek session.

Stacking two quality days back-to-back. Hard Tuesday + hard Wednesday (with intent to "get them out of the way") produces a poor second session and accumulates fatigue. The 48-hour minimum between hard sessions is the standard for a reason.

Periodising speed work across a training block

The most effective use of these three workouts across a 16-week build varies by phase:

• Weeks 1–4 (base): Fartlek introduces speed. One session per week, building from 20 to 35 minutes of mixed-intensity work.
• Weeks 5–8 (early build): Tempo runs added. One tempo + one fartlek per week, separated by 72 hours.
• Weeks 9–12 (build): Intervals introduced. One tempo + one interval session per week. Fartlek drops to occasional fill-in.
• Weeks 13–16 (race-specific and taper): Workouts narrow to the energy systems most specific to goal race. Volume drops; intensity holds.

This is the rough Lydiard-Daniels lineage that underpins most modern distance plans. The progression matters: introducing intervals too early in a build, before the tempo base is established, leads to rapid plateau and elevated injury risk. The progression from broad (fartlek) to specific (intervals at race pace) is part of why periodisation works.