I did my first half marathon a couple of weekends ago, and my big toenails have turned a gnarly shade of purple/black, and one is lifting away.
I was fitted for my shoes, but I think that the heat on the day probably made my feet swell more than usual and my shoes came loose. Not sure. Have had the same shoes (2 pairs of) for a year. Hadn't considered the socks, but they are a bit on the biggish side.
Am resigned to the fact I am about to lose my toenails in time for the summer. Will be trying to tie my laces tighter perhaps, and perhaps some different shops. My toes don't hurt now after 10 days, but look horrid!