Fatrtlek runs are workouts which mixes low intensity runs with very high intensity runs. I basically walked at a slow pace (about 61% MHR) for a few laps then started to run at maximum speed until I reached 85% MHR. Once I did that I would slow down again to get my heart rate to 65% and continue the whole cycle. It’s different from interval runs as it is very unstructured and depends on how my body feels instead of going all out no matter how my heart rate is.
I think it’s more enjoyable than intervals and maybe more beneficial as it disciplines me to pay more attention to how my heart feels as I made sure I don’t sprint until I get the heart rate back to 65% MHR.
Today I did for about 4km or about 40 minutes. Next time I do it I am going to modify it a bit and time how long it takes me to recover from 85% to 65% MHR. Recovery is a great indication of my fitness level.