Fartlek running involves varying your pace throughout your run, alternating between fast segments and slow jogs. 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, and to experience changes of pace. Many runners, especially beginners, enjoy fartlek training because it involves speed work. But it is more flexible and not as demanding as traditional interval training. Another benefit of fartlek training is that it doesn't have to be done on a track and can be done on all types of terrains -- roads, trails, or even hills.
To do a fartlek workout, try introducing some short periods of slightly higher pace into your normal runs. Maintain the faster pace for a short distance or time intervals, such as 200m or 30 seconds. The intervals can vary throughout the workout, and you can even use landmarks such as streetlights or telephone poles 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 incorporate more slightly fast intervals later in the run.
Fartlek training puts a little extra stress on your system, eventually leading to faster speeds and improving your anaerobic threshold.