some engineering sucks wrote:
Spiderman?
I think the face mask would be hard to run with, but the tight fitting clothing is ideal, especially if the race is in cooler weather. You might be able to do well in a superman costume if you can find a lightweight cape. I also like the tarzan idea for a warm weather race.
Adventure character is an interesting theme. Since you're not limited to fictional adventure characters, you might consider going as Steve Fossett. If you remember, back in 2002 he became the first person to circumnavigate the earth in a balloon on a solo flight. You could tie a bunch of helium balloons to yourself to make yourself lighter and faster. The increased drag may offset the reduction in weight however.