'Runner of fartleks'. Above, has defined it as well as could be. I ran a lot in the 70s and definition 1 was what everyone meant by the term then. I also will point out that I do know anyone who realy did this regularly. I might have like twice in all of HS (I was not on an official school team and therfore worked out as I saw fit).
Fast forward several decades and it's now a popular thing. Or at least a popular term, since one could easily argue that definition 2, which is what everyone does now (but usually not on a track, in my impression) is not really fartlek.
I think for you, looking at mid distances, the most useful way to integrate this concept is the long run with accelerations or surges throughout. If you going really long you may not be psyched on this but a run of an hour or hour and a half can be done as follows:
First 15 min WU
Accelerate to something around race pace and hold it for several seconds.
Cruise another 5 min.
Accelerate again. These can get gradually faster and/or longer, but if they're more than 30 second. It becomes a different type of workout. Also, they probably shouldn't really get to 100 percent full speed, just approach it but don't squeeze out that last couple percent.
Do a dozen surges total. Maybe more or less.
Last 15 CD
I recently did the following:
3 min hard. 1 min easy.
Do this 10X
2 min hard. 1 min easy
Do this 10X
1 min hard. 1 min easy
Do this 10X
The 2 mins were slightly faster than the 3s and the 1s a little faster yet. The minutes 'off' got slower but really shouldn't. The idea is that you still cruise but don't really jog. This was on road with no mile markers. Just by feel. Great workout; 90 mins, 60 hard. However, this is probably better if you're training for something like 15k or longer. Not as much of a 1500 type workout, but who's to say some of these 15/5k guys don't do this kind of thing? I know marathoners do.
Hope something in here is informative and helpful. Good luck!l