Like others I think the cramp issue is partly about doing things you're less used to. I do long runs (30+ miles) on my own without ever getting cramp, but they're mostly fairly easy paced, at most steady. However, I've recently had twinges of cramp on shorter (5-10 mile) races. I don't really understand it (was always adequately hydrated, salted, etc.) but can only assume it's because I'm running much harder/faster, putting the muscles under slightly different stresses. [I'm not denying longer runs and hotter conditions will also be a factor sometimes, clearly they are.]