I run a few races a year, and tend to be on the slower side of things, at least for half and full marathons.  On races that allow it, I've stopped sitting back in the slower corrals, because nobody respects them.  I am so tired of being stuck behind herds of walking planets who lined up in the waaaaaay wrong corral (sir, I don't think your 400 pound ass is cranking out 8 minute miles anytime soon).  What's funny is I still only get passed by a scant handful of people, even moving up a full minute's pace from what I plan on running.  I want to murder anybody who starts off walking in the 10 min/mi and up corrals.  They have absolutely no business being there, and I once got yelled at because I ran full tilt into a walker because a guy ahead of me skirted around it at the last possible second to avoid a collision of his own.  There just wasn't time to react.  Keep in mind the walker had started in the 8 min/mi corral.  I would love to run a Disney race, but still haven't done it because I've heard it is 90%+ walkers and planets.  What's the point.  Just buy a medal from the gift shop, assholes.