Lots of runners lose toenails in training and events. You might want to look on the Runners World forum for info on this (perhaps there is also mention here - maybe look for "black toe" or something.)

My marathoner friends were always losing toenails. (It was kind of a sick badge of honor). When I did the last marathon I lost both big ones as well, but I chalked that up to being on coumadin at the time.