What it really takes to succeed as a collegiate cross-country runner is discipline...and a body frame that doesn't make you wonder whether you should play left guard for the football team. (There are definitely some taller, heavier folks excelling at marathons out there, but the top runners in the world almost never have an ounce of weight to spare.) 

If you're a serious runner and a hard worker, there's probably a spot for you at a D-II or D-III program. It's even possible to walk on to the junior varsity team at a D-I program if you've had a conference-caliber career in high school.

But the varsity team has only seven runners. Needless to say, it's a bit harder to get picked for that team.