In essence, a fartlek workout is a running interval session without the track and the accompanying structure to the workout. The idea is to sandwich harder, faster efforts in the middle of a training run on the roads.
These harder efforts can last last anywhere between 15 seconds to three minutes, and should vary in distance and intensity throughout the run. The longer efforts will be slower, while the short efforts will be faster.
If you are training for longer races, the harder efforts should tend towards longer duration, while shorter efforts are good for shorter races. When running these harder efforts, it is best to spot a landmark in the distance such as a tree or telephone pole and run hard to that point. Start out with a pace that you will be able keep constant for the entire hard effort, then run easy before another hard effort. The recovery periods will vary along with the hard efforts, but in each case make sure you have recovered adequately before starting the next hard effort. A twenty to thirty minutes session (hard efforts plus the recovery time in between) is plenty.
A fartlek session can be as easy or as hard as you want to make it. If you are new to speed training, it is best to err on the side of caution and start out easy for a few weeks. Remember to warm up with ten minutes of easy running before the hard efforts, and allow for a ten minute easy run at the end as well, and pace yourself throughout the workout. Don't start out too fast and run out of gas before the end of the workout. You want to finish feeling strong.
While this workout can be as easy or as hard as you want to make it, it is advisable to limit your speed workouts to no more than one a week. For best results, vary the type of speed workouts you do.