Fartlek running can be structed or completely random. The fartlek session is running with added surges, once you have completed the surge, your aim is to return to the previous pace you were just running at.
Such training can be excellent in races, imagine for example you are racing besides someone and you are able to suddenly increase the pace for a set period and then drop back to the pace you were just at.
After a few surges, your opponent will feel the pressure and let up, giving you control of the race.
A popular fartlek session in Australia is the mona fartlek which involves – 90seconds x2, 60seconds x4, 30seconds x4 and 15seconds x4. This session equals 20 minutes. So after warm up/down you are looking at covering up to 40 minutes of training!
You can also pick landmarks, sprint between telegraph poles or streets, the choice is completely up to you.
Some further examples-
#90seconds x10 with 90seconds recovery totaling 30 minutes.
#1minute on/off, 2minutes on/off, 3minutes on/off 4minutes on/off, 3min/on/off, 2minutes on/off, 1minute on/off totaling 32minutes.
Also you can try to remember how much distance you travelled, then try to beat that doing the same session next time. For example, the mona fartlek people try reach 5km or if your much more experienced, 5.5km+.
You can vary it however you choose, the important part is aim to be running at a decent pace for your off and then surge on top of that for the surges.
Fartlek sessions being high intensity should only be done once fortnightly to maintain the session quality. A massage the following 1 or 2 days will help your body recover quicker which will get you those big PB’s sooner!