One significant factor, I think, was that running (and racing, though there weren't many races) was something you could do by yourself, or with others -- it was up to you. No varsity team to make, no coaches, no real organization in most places (if you don't count the AAU).
And for me, at least, the fact that it wasn't team-based was a big part of its draw. I didn't like team athletics and had avoided them in HS and college. This was different.
I was in my mid thirties in the mid seventies. I think there were lots of guys like me -- not exactly hippies, not exactly counter-culture guys, but definitely not mainstream league baseball/softball/basketball/hockey kind of guys. But we liked to test ourselves athletically, and running against the distances and each other ... the whole process was important.
I still feel the same way.
It's been a long time since then but if my memory isn't tricking me, none of the dozen or so guys I regularly ran with had been HS or college athletes of any kind.
Many of us ended up joining running clubs, though. But that seemed different from organized leagues.
It's not as though we were a bunch of revolutionaries or anarchists. Don't get me wrong. But we were all strongly attracted by the freedom of the running community. Nobody really cared who you were. If you liked to run, you could find plenty of companionship (and competition, when it came to that) in our park.
And by the way, I'm an anthropologist so I know all about, and support, participant-observation, oral history, unstructured interviews, and all the rest. Go for it.