I too have seen this on a couple of other forums.

Folks that are out to break records are generally 'findable' --- most who set out to do so announce same ahead of time. Records broken on both the PCT and AT last year are good examples.

"... or some sort of milestone" is more vague and leaves lots of options. There certainly are interesting folks out there, like the guy who wore a wedding dress all along the PCT (a different one for each section).

Probably best place to post this in the "some sort of milestone" context these days would be on the per-year-class-of pages for each major trail on Facebook.

I will say that when you say first that you're developing a show about thru-hiking but then say that you want folks out to break records and you're looking for "the most intense thru hikers" --- that suggests to me that you're not at all looking to capture what thru-hiking is about in the more general sense, but rather looking for those on the extreme edges. Like the guys that took the CDT route last year that hit some improbable number of 14'ers along the way.

Nothing wrong with that, certainly, but I would say that rather than "a show on thru hiking", you're looking for a "a show on extreme thru hiking". Best to talk then with the handful of folks that have done anything like that.

The legends shouldn't be hard to find --- 2013 record breakers for a start, then others like Tatu Joe, Eric D, Scott Williamson, Flyin' Brian, Justin Lichter, Andrew Skurka ... it's a small community and I would guess they mostly know each other somewhat and are aware of who is up and coming to challenge the records in future.