If you're new to Fartleks, it means "speed play" in Swedish, a type of workout that alternates between slower and faster running or playing around with your speed. If I tell you that you're very farty, you know what I really mean!
By alternating between an active recovery pace and 10k pace, for example, fartleks help build your aerobic capacity or endurance. After some dynamic stretches and an easy warmup, a traditional fartlek workout may involve 1 minute on (i.e. 10k pace) and 1 minute off (i.e. active recovery pace).
One variation I have been incorporating is a 4 minute effort where you start with your marathon pace for 1 minute, increase to your half marathon pace for the next 1 minute, ease back to your marathon pace for 1 minute, and then finish with your 10k pace for the final minute. 2 minutes of active recovery. Repeat 5 times. Play around with that final minute, finishing faster each effort with your 5k, 1 mile, or 400m pace.
I love fartleks because they make you quickly adapt to different speeds and become a more efficient runner. This will condition you to become faster over a longer distance. You train your mind to become stronger and not give up, so you can push through any mental block as you master knowing and varying your various pace zones.
Finally, fartleks are fun! Hence the "play." They can also mimic the intensity of a race, e.g. if you're running in a pack and a few people surge ahead and you want to stay with them or if you want to lead a surge yourself!