Fartlek workouts are not only fun to say outloud but they are fun to run. Fartlek is Swedish for "speed play" and that is exactly what it's all about. Fartleks are a form of interval or speed training that can be effective in improving your speed and endurance.
Fartlek running involves varying your pace throughout your run, alternating between fast segments and regular slow running. Unlike traditional interval training that involves specific timed or measured segments, fartleks are more unstructured. Work-rest intervals can be based on how the body feels. With fartlek training, you can experiment with pace and endurance while experiencing different changes of pace. It is more flexible and not as demanding as traditional interval training. Another benefit of fartlek training is that it can be done on all types of terrain -- roads, trails, or even hills.
After warmup period of running, you play with speed by running at faster efforts for shorter period of time (for about 30 -- 35 seconds) followed by an easy-effort running to recover. The intervals can vary during the workout and you can use also landmarks such as streetlights, telephone poles, street signs to mark your segments.
Once you complete a fast segment, slow your pace to below your normal running pace, until you have fully recovered and your breathing has returned to normal. Then return to running at your normal pace and continue to incorporate fast interval segment in the run.
It's fun in a group setting as you can alternate the leader and mix up the pace and time. This way you reap the mental benefits of being pushed by your running partners through an unpredictable workout. The goal is to keep it free-flowing so that you run at harder efforts but not a specific pace.
Fartlek training puts a little extra stress on your system, eventually leading to faster speeds and improving your anaerobic threshold.