[smeyer]

Like the previous commenter mentioned, that's mostly water. I've seen various numbers between 100 and 150 calories burned per mile for runners, which at the high end is just a bit over one pound's worth of calories for a marathon.

[garmaine]

I’ve read about people who do things like swim the channel, and bulk up (10’s of lbs) before doing so. Given that they lose this weight in the process, how does that happen?

[smeyer]

I think that's mostly independent. A swimmer bulking up for a channel swim is probably putting on a mix of water, fat, and muscle in the slow lead up to the swim (and maybe extra water in the days before) and then losing water during the swim. Here's [0] a discussion of swimmers strategizing about weight gain, and you can see that it's largely about issues like insulation from cold, buoyancy, and strength, and not because they're worried about losing the weight during the swim itself.

[0] http://marathonswimmers.org/forum/discussion/90/when-to-gain...