I know lung capacity isn't the limiting factor in running performance, but I think holding your breath might help in a different way.
When I see how long I can hold my breath under water, I usually start out at only 20 seconds or so. The next time is 30, the next time is 45, and the next time is about a minute, and then I level off. I figure holding one's breath is more about teaching your body to relax and having your brain realize you're not going to die if don't breathe right away. For anyone who's read "Brain Training," it's kind of the same concept. I figure teaching yourself to hold your breath longer would let you push yourself farther in maximum efforts.
Opinions?