It's geared towards competitive running so there's a sizable hs/college crowd, but there are also a lot of coaches and former competitive athletes in there too. Definitely a lot of off-topic stuff but you can filter that out. If you're on the message board page, click the "currently viewing" drop-down menu in the upper left to get where you want.

Overall exercise forums do seem to be weirdly specific

I think it's because fit people tend not to be passionate about fitness, but about their specific athletic endeavors.