It's not necessarily that runners are all effeminate (although a lot are).
It's more that they're extremely elitist, with no good reason to be that way. Runners are essentially at the bottom of the athletic ladder (well, maybe curlers or bowlers or what have you, but you get the point). Most have subversive (or overt) inadequacy issues (I was there once, too) and feel the need to constantly assert their perceived superiority.
I knew a bunch of my teammates in college who loved to make fun of those "dumb jock" football players (which was stupid since it was an Ivy school and the football players all seemed fairly bright). These guys reveled in the fact that they were all man enough to go out and run 80-120 miles per week and that made them men.
Everyone has a different definition of what's "masculine", but many runners don't fit society's standard criteria all that well. If that's your nature, hey, no worries.
But if you actively avoid things that society perceives as "masculine" such as football, etc, just because you think you're somehow "better" than that, well that's when you cross the line over into the category of giant tool.