Most runners reach a point where easy miles stop being enough. You want to get faster. You start looking into speed training. And immediately you find two terms used interchangeably by some coaches and treated as completely different things by others: fartlek and interval training.
They are not the same. They produce overlapping benefits through different mechanisms, suit different training phases and different types of runners, and each has situations where it is the smarter choice. Here is a clear explanation of both, the differences that actually matter, and a straightforward guide to when to use each one.
What Is Fartlek Training?
Fartlek is a Swedish term meaning speed play. It is a style of continuous running in which you vary your pace based on feel, terrain or personal preference rather than a fixed schedule.
In a fartlek session you run the entire time. There are no full stops. When you push hard you recover with an easy jog, not by standing still. The transitions between fast and easy happen when you decide, not when a timer goes off.
A basic fartlek session might look like this: warm up for ten minutes, surge hard to the next corner, jog until your breathing normalizes, surge hard again up the hill, jog across the flat, repeat for twenty to thirty minutes, cool down.
The pace of each surge, the length of each recovery, the number of repetitions — all of this is self-governed. That is what makes fartlek what it is.
What Is Interval Training?
Interval training is structured speed work in which periods of hard effort alternate with defined rest or recovery periods. Unlike fartlek, the work and rest are predetermined before the session begins.
A standard interval session might look like this: warm up for fifteen minutes, run 400 meters at your 5K race pace, rest for 90 seconds, repeat six times, cool down.
Every variable is fixed in advance. The distance of each rep. The target pace. The length of the recovery. The number of repetitions. You follow the plan, not your instincts.
Interval training is typically done on a track or flat measured course where distances can be controlled precisely. The recovery periods can be complete rest (standing or walking) or active rest (slow jogging), depending on the session design.
The Core Differences
Structure
This is the fundamental distinction. Fartlek is unstructured or loosely structured. Interval training is precisely structured. Everything else follows from this.
Recovery type
In fartlek, recovery is always active. You jog between surges. Your heart rate stays elevated throughout the session. In interval training, recovery can be passive (complete rest) or active, depending on the program. Complete rest allows more intense efforts per rep but accumulates less overall cardiovascular stress.
Where you do it
Fartlek works on roads, trails, grass, hills, or a treadmill. You do not need a measured course. Interval training is typically done on a track or a measured flat route so you can hit precise distances and track pace accurately.
Pace control
Interval training targets specific paces tied to race goals or physiological thresholds. If your coach prescribes 800 meter repeats at 5K pace, you run that pace as accurately as possible. In fartlek, pace is governed by effort and feel. There is no target number. You run hard, you run easy, and your body decides where those boundaries sit on a given day.
Mental load
Interval training requires focus and discipline throughout. You are executing a plan, hitting targets, tracking reps. This is useful but also demanding. Fartlek has a lower cognitive load. You are not managing a plan. You are reacting to how you feel and what is in front of you.
What They Have in Common
Both methods build speed and endurance. Both stress the aerobic and anaerobic systems. Both improve VO2 max and raise lactate threshold over time. Both are more effective for developing running performance than steady-state easy running alone.
The differences are in how they achieve these adaptations and which situations each one suits best.
When Fartlek Is the Better Choice
Early in a training cycle
Most coaches use fartlek in base-building phases before introducing formal track work. The effort-based nature lets athletes develop a feel for faster running without the risk of overreaching on pace. You build the habit of surging before you start targeting specific pace zones.
When you are returning from a break or injury
Fartlek is self-regulating. If your legs feel flat, your surges will naturally be shorter and slower. There is no pace target to miss, no rep count to hit. The session adjusts to your actual fitness level on the day.
For mental freshness
Late in a heavy training block, the track can feel relentless. A fartlek session over trails or roads provides the same speed stimulus with a completely different feel. Many experienced runners use fartlek every three to four weeks as a deliberate break from the precision of interval sessions.
For trail runners
Fartlek originated in the forests outside Stockholm. Running hard up a hill, recovering on the descent, surging across a flat section is exactly what fartlek was designed for and exactly what trail racing demands. Trying to run prescribed intervals on technical terrain is impractical. Fartlek fits naturally.
For beginners
There is no pace to miss, no rep count to stress over, and no track to navigate. The effort-based approach makes fartlek almost impossible to do wrong. If you go too hard, you recover longer. If you go too easy, you feel fine and run again. It is a forgiving entry point into speed training that delivers real benefits without the intimidation of a structured session.
When Interval Training Is the Better Choice
Race-specific preparation
If you are training for a 5K, you need to spend time running at 5K pace. Interval training is the most direct way to accumulate that time. Fartlek surges at approximately the right effort are useful, but precise repetitions at race pace build the neuromuscular and metabolic adaptations that convert directly to race performance.
VO2 max development
Research consistently shows that structured high-intensity intervals are among the most effective stimuli for improving VO2 max. The precise pace targets and controlled rest periods allow you to accumulate significant time at near-maximal aerobic intensity, which drives cardiovascular adaptation efficiently.
Tracking progress
If you want to measure whether your fitness is improving over time, intervals give you data. Running 6 x 800 meters this week and comparing your times to the same session four weeks ago tells you something concrete. Fartlek tells you how you felt, which is valuable but harder to quantify.
Working with a coach
Most structured training programs use intervals because they are prescribable and measurable. A coach can assign 10 x 400 meters at a specific pace and know exactly what physiological stimulus you received. Assigning a fartlek is harder to verify and calibrate from a distance.
A Side-by-Side Comparison
Structure Fartlek: unstructured or loosely structured by feel Interval training: precisely structured by distance, pace and rest
Recovery type Fartlek: always active (jogging) Interval training: active or passive depending on the session
Location Fartlek: anywhere Interval training: track or measured flat course
Pace control Fartlek: effort-based, no target pace Interval training: precise pace targets
Mental load Fartlek: low, reactive Interval training: high, disciplined
Best phase Fartlek: base building, recovery weeks, early season Interval training: race-specific preparation, peak training
Best runner Fartlek: beginners, trail runners, returning runners Interval training: competitive runners targeting specific races
Progress tracking Fartlek: difficult to quantify Interval training: precise and trackable
Do You Have to Choose?
No. Most effective training programs use both.
The typical approach is to use fartlek during base-building phases when the goal is developing aerobic fitness and a feel for faster running, then transition to structured intervals as a race approaches and race-specific pace work becomes the priority. Many coaches also reintroduce a fartlek session every few weeks during peak training to give athletes a mental and physical break from the track.
Thinking of it as either-or misses the point. They are tools that solve different problems at different points in a training cycle. A good running program uses both strategically rather than committing to one exclusively.
The Short Version
Fartlek is continuous running with unstructured pace variation governed by feel. Interval training is structured speed work with defined distances, paces and rest periods. Fartlek is more flexible, more accessible and better suited to base building and trail running. Interval training is more precise, more measurable and better suited to race-specific preparation and VO2 max development. Most runners benefit from using both.
Sources
Wikipedia. Fartlek. Last updated 2024.
Runners Vault. Tempo vs Intervals vs Fartlek. runnersvault.com.
Billat, V. (2001). Interval training for performance: A scientific and empirical practice. Sports Medicine, 31(1), 13-31.
Foster, C. et al. (2015). The effects of high intensity interval training vs steady state training on aerobic and anaerobic capacity. Journal of Sports Science and Medicine.
