Watched many workout videos of many pro groups that do fartleks (time based) I honestly don't see the point for people at that level many do them on a track which I find even weirder. if you are running a time based work why do it in small loops instead of in larger loops or straight lines
Here is my view what on fartlek's are good at:
1. Good for a runner who would get upset with running slow workouts early in a build for a season or for a workout between seasons.
2. Good if you don't have access to a track or measure trail/loop hence my not understanding doing them on a track and finishing and starting on random places on that track.
3. Good for new runners for them to learn how to run easy and fast and progress slowly in workouts or at least not digress.
4. Good for lazy coaches - I admit this is me really easy to assign a fartlek instead of trusting gps or hoping your athlete has access to a track.
Basically tempos are far better at working on the aerobic side and intervals are great for the anerobic threshold fartleks fall in the worst of both worlds where it's not the benefits of a tempo/aerobic threshold workout with all the breaks of and none of the benefits of anerobic threshold either as the paces tend to be someone between the thresholds or closer to aerobic but with rest instead of no rest. They also give coaches zero gage as to what type of shape the runner is in since it's hard to estimate the shape an athlete is in because they ran 3 min hard 2 min easy or medium. the overall pace isn't very helpful and will be quiet a bit slower than a tempo.
So my point is a pro runner or "a mature runner" should be mature enough to do a tempo or progression run into a tempo and be ok with running "slow" compared to when they are further along in their build and not do any fartleks or just do intervals of measured distance with measured rest (distance) to give themselves and coach realistic expectations of what will happen on race day as they start doing more sub tempo work (track workouts).
One could argue me doing an alt mile workout is a fartlek but:
1. I rarely do those
2. That workout being distance based and longer gives me a better idea of what shape I am in (short reps on a GPS are very inaccurate distance wise)
I personally haven't done a fartlek in a decade and yet have improved consistently because I am totally ok with my tempo pace being 20-30 sec/per mile slower depending where I am in a marathon build or a track season build.
Did I miss something they are good at?