A friend of mine was saying the other day that he rides regularly with another guy who's a racer (but I don't know what level). At shorter distances my friend gets schooled by the guy. However at distances greater than about 50 miles, my friend always beats this guy. I noticed the same thing when I first got into cycling. The people I rode with were always faster than me at anything <30 miles. However at much longer distances, 100+ miles, I could always finish ahead of them or in some cases finish something that they dropped out of. I've heard a lot of people talk about this sort of thing. What explains these differences? Is it the types of muscle fibers you have? ie. fast twitch vs. slow twitch.