Fartlek

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.”^[2] 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.
Steve Moneghetti 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.
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.
Fartlek is down to your imagination and need during that specific periodisation of training.