You can experiment with pace and endurance, and experience changes of your pace. It can be done on a track and all types of terrains such as roads, trails and hills. You can do these with a group and alternate leaders to add variety of the pace and time/distance.
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.
Short fartlek session:
Run easy for 10 to 20 minutes to warm up.
Follow with four to six 20-second pickups, jogging 40 seconds in between each pickup.
Perform 2 to 5 sets of 2 minutes, 1 minute, 30 seconds, starting at 5K race effort for the 2-minute interval and increasing the intensity for the 1-minute and 30-second segments. Adjust the effort level of each interval, as well as the number of sets, depending on your experience and training focus.
For recovery, jog one minute after the 2-minute interval and 30 seconds after the 1-minute pickup. Take a 2-minute jog for recovery in between sets.
Cool down with 10 to 20 minutes of easy jogging.
The key is adjusting your effort level, along with the number of sets, to suit your individual needs.
Have fun with it.