Fartlek workouts have always interested me. Traditionally they are a way of running a randomised interval workout - using landmarks, like say, a letterbox or parked car, to kick off the intervals.
"Run hard between red cars" or "Run hard between post boxes"
The randomness of your environment resulted in intervals of variable lengths, with the runner not knowing when the interval will end or how long they will have to recover before the next one.
To mimic this for those of us that are stuck running on bike paths or running tracks, or just want a more technically advanced version, I've built Run Randomly.
Run Randomly generates a workout with various interval lengths. There's no variation in interval pace - it's "run easy" or "run hard". The workout will alternate between the two after a short warm up and allow for a cool down at the end. You provide a duration and target speed (just use your 5km pace for your first one), then a workout is generated for you.
You can upload a workout directly to Intervals or download a FIT file that can be uploaded to most running watches. For the most authentic experience, avoid peeking at the workout before you hit the start button.