Fartlek, which means "speed play" in Swedish, is continuous training with interval training. Fartlek runs are a very simple form of a long distance run. Fartlek training “is simply defined as periods of fast running intermixed with periods of slower running." For some people, this could be a mix of jogging and sprinting, but for beginners it could be walking with jogging sections added in when possible. A simple example of what a runner would do during a fartlek run is “sprint all out from one light pole to the next, jog to the corner, give a medium effort for a couple of blocks, jog between four light poles and sprint to a stop sign, and so on, for a set total time or distance." The variable intensity and continuous nature of the exercise places stress on both the aerobic and anaerobic systems. It differs from traditional interval training in that it is unstructured; intensity and/or speed varies, as the athlete wishes. Fartlek training is generally associated with running, but can include almost any kind of exercise.
Swedish coach Gösta Holmér developed fartlek in 1930, and, since then, many physiologists have adopted it. It was designed for the downtrodden Swedish cross country running teams that had been beaten throughout the 1920s by Paavo Nurmi and the Finns. Holmér's plan used a faster-than-race pace and concentrated on both speed and endurance training.
First fartlek sessions
This is the first session that was designed by Gösta Holmér for a cross country (multi-terrain) runner. This is also an example of what a fartlek session might look like, but fartlek sessions should be designed for an athlete's own event or sport, as well as catering to their individual needs. Sessions should be at an intensity that causes the athlete to work at 60% to 80% of his or her maximum heart rate. This should mean that the body will not experience too much discomfort while exercising. An athlete should also include a good warm up at the beginning of the session, and a cool down at the end of the session, to improve performance, minimize post-workout muscle soreness, to decrease the chances of injury and for other reasons.
- Warm up: easy running for 5 to 10 minutes.
- Steady, hard speed for 1.5–2.5 kilometres (0.9–1.6 mi); like a long repetition.
- Recovery: rapid walking for about 5 minutes.
- Start of speed work: easy running interspersed with sprints of about 50–60 metres (160–200 ft), repeated until a little tired.
- Easy running with three or four "quick steps" now and then (simulating suddenly speeding up to avoid being overtaken by another runner).
- Full speed uphill for 175–200 metres (570–660 ft).
- Fast pace for 1 minute.
- The whole routine is then repeated until the total time prescribed on the training schedule has elapsed.
Runners hypothetically “run hard for two mailboxes, recover for three, run hard for three, recover for two." When executing this type, the runner continues like this for the allotted time or distance determined.
Dog park version
Runners speed up when they get close to a dog in order to pass them; after passing the dog, they would then slow down for the recovery period. 
Many runners use music while they run.  Runners can use their music as a template for their fartlek run by changing their speed according to different parts of a song. For example, they can speed up during the chorus and slow down for the rest of the song.
Three speeds version
In order to add more variety and complexity, runners can add another speed into the run. Within any run, “there is no reason why three different paces should not be included." This would change a normal fartlek by doing a jog, run, and a full out sprint.
Steve Moneghetti (Mona) devised this session with his coach Chris Wardlaw over the phone back in 1983 when he was just 20. He wanted a solid fartlek session, one that would help improve his speed as well as endurance and stimulate an ability to change pace mid-run, something that helped later on his career when competing against African athletes, who tended to speed up mid-race. A session would consist of: 2×90sec, 4×60sec, 4×30sec, 4×15sec with a slower tempo for recovery of the same time between each repetition. The session takes 20 minutes in total.
Since this workout is very easily manipulated, “fartlek training allows you to add an endless variety of intervals to your aerobic workouts, which helps to keep you stimulated." A great plus to performing this workout is the variety it adds to fitness regimens; it is possible to change the amount of distance, time, fast bursts, recovery periods, and even the time at which you do each component.
Race day readiness
For competitive runners, fartleks and all other types of interval runs are the key for race day readiness. The alternating speeds that are the defining point of fartleks allow runners to work “both the aerobic and anaerobic training systems while simulating the ebb and flow nature of competitive running." 
Fartleks keep runners' bodies at top shape so that they can keep racing. Putting fartlek runs in a workout routine keeps “muscles, tendons, nerves, etc. used in running going at top capacity." In other words, regularly implementing fartleks keeps your body strong enough to maintain the mechanics of racing.
By alternating the “intensity of your workouts, you will burn more calories than you would by keeping a steady pace." While running, the runner's body uses a combination of carbohydrate and fat, with relatively more carbohydrate metabolized at faster speeds and relatively more fat the longer the workout lasts. A fartlek workout will allow the body to adapt to using both sources of energy, with the desired adaptation towards fat metabolism occurring during slower periods. In addition, varying speeds improves cardiovascular endurance slightly better than running at a steady pace for the same time and total distance.
Sports training variability
Fartleks can be specially tailored to fit the needs of different types of athletes. An example includes basketball, where “you must exert maximum effort while running a fast break, while you exert very little effort while standing at the foul line." It is possible to alter the type and timing of the fartlek in order to mimic the intensity of an average basketball game. This can be done for other sports such as tennis, rugby, soccer, and football as well.
Comparing other running exercises
Comparable runs include the traditional interval training, tempo runs, and ladders. These workouts are very similar to fartleks, but there are slight differences that distinguish one from another.
Tempo runs are typically run for 20 to 25 minutes at a 6 or 7 RPE (out of 10).  This exercise is “like an Oreo cookie, with the warmup and cooldown as the cookie, and a run at an effort at or slightly above your anaerobic threshold (the place where your body shifts to using more glycogen for energy) as the filling." What runners do here is warm up at a slow and steady pace, then run harder than they would on a normal distance jog for an allotted amount of time, and then do a cool down with a very similar speed to the warm up.
Intervals “are short, intense efforts followed by equal or slightly longer recovery time." By the end of a short burst of speed, the runner is barely able to keep up that pace. Unlike fartleks, interval runs are much more structured; usually running the fast and slow spurts at exact times or distances. Interval runs and tempo runs differ in the fact that tempo runs maintain a slightly fast pace for a set amount of time, while interval runs consist of alternating between sprints and slow sections instead of keeping one speed.
This workout is defined as “a speed workout in which the fast parts vary in length." Basically, athletes will run a small amount at a hard pace, work their way up, and work their way back down with timed breaks in between. Ladders are similar to interval training in that they require more structure, but they are different because the faster speed sections vary in time or distance. Although ladders are most similar to interval runs, the fact that the portions performed at the faster speed vary in length or time mirrors fartlek runs.
- McArdle, William D.; Katch, Frank I.; Katch, Victor L. (2009) . "Training for Anaerobic and Aerobic Power". Exercise Physiology: Nutrition, Energy, and Human Performance (7th ed.). Lippincott Williams & Wilkins. p. 483. ISBN 978-0-7817-9781-8. Retrieved May 7, 2012.
- Kerkman, Jill. "What the Fartlek?!" Breaking Muscle, 2012. Web. http://breakingmuscle.com/running/what-the-fartlek
- Barker, Jill (29 March 2011). "Making the leap from lazy jogging to real racing". Vancouver Sun. Archived from the original on 4 April 2011.
- McDonald, Lyle (1998). The Ketogenic Diet: A Complete Guide for the Dieter and Practitioner. p. 247. ISBN 978-0-9671456-0-0.
- Schatzle, Jr., Joe (November 2002). "Finding Fartlek: The history and how-to of speed play". Running Times Magazine. Archived from the original on 2012-03-16. Retrieved 2011-04-05.
- Schatzle, Jr., Joe (November 2002), Runner's World, "Finding Fartlek"
- Shaw, Jené. "9 Ways To Fartlek." Triathlete.com. N.p., 11 Feb. 2014. Web. http://triathlon.competitor.com/2014/02/training/9-ways-to-fartlek_51140
- Bean, Adam. "Running With Music." Runner's World & Running Times. Runner's World, 1 Dec. 2010. Web. http://www.runnersworld.com/workout-music/running-music-0.
- Mackenzie, Brain. "Fartlek Training." BrainMac Sports Coach. N.p., 10 Oct. 2014. Web. http://www.brianmac.co.uk/fartlek.htm
- Hutchins, Michael. "What Are the Benefits of Fartlek Training?" LIVESTRONG.COM, 21 Oct 2013. Web. http://www.livestrong.com/article/471208-what-are-the-benefits-of-fartlek-training/
- Galloway, Jeff. Cross-country Running. Aachen: Meyer & Meyer Sport, 2011. Print.
- Santos, Larissa (14 June 2016). "Which Is Better between Running Faster or Longer?". Avenue Form. Archived from the original on 11 August 2016. Retrieved 23 June 2016.
- Hadfield, Jenny. "What's the Difference Between Fartlek, Tempo, and Interval Runs?" Runner's World & Running Times. N.p., 21 Nov. 2012. Web. http://www.runnersworld.com/race-training/whats-difference-between-fartlek-tempo-and-interval-runs
- Rodgers, Bill, and Scott Douglas. The Complete Idiot's Guide to Jogging and Running. New York, NY: Alpha, 1998. Print.