Toenail injury possible from up-tempo distance running?

I went running on Sunday for about 6 miles (10 km) at a 7:30-minute mile pace. Since then, the nail on my big toe on my right foot has been incredibly sore. Sometimes it feels loose. What causes this? Any remedies? I'm going to be a bridesmaid this weekend; should I avoid wearing high-heeled shoes?

Your shoes are too tight. This is a common injury among long-distance runners. I don't want to scare you, but in extreme situations you the toenail can turn black, undergo necrosis and eventually fall off. Permanently. You should buy running shoes which are half the size bigger than your regular shoes (i.e. 8.5 if your regular size is 8)

Also running downhill can exacerbate this condition. Keep the toenails clipped too. In the meantime, take it easy and don't run for few days until the pain goes away.

Really dedicated runners sometimes get the big toe-nail permanently removed! Just because it missing anyway during their running season, they just go to a doctor and get rid of it once and for all.
On the bright side, they claim that they can put nail polish on the skin where the nail used to be and no one notices when they wear sandals.
Either that or they're so used to their feet being grody that they don't even know what's normal or not anymore.

Yucks. I used to get that all the time running downhill. Just cut your nail really short, but paint over some parts of your toe itself, no one will really notice if you use dark red or something strong.