What do marathon runners do when they have to pee?

[DCopper04]

I'm not thinking about the usual marathons. I'm thinking about the 100-mile marathons that take like 19 hours to complete (if you're fast). I know that they sweat out a great deal of fluid during the run, but surely they've hydrated themselves very well beforehand, and they drink a whole lot of fluids during the race too. So what the hell do they do? Can they really hold it in for 19 or more hours? Or do you think a lot of runners just let it go during the race?