I had a bloody toe once and I wasn't quite sure how or where it was coming from either! I think it was just extra friction from a long run. So I don't know much about losing toenails but I would just make sure to wear a thin but comfortable pair of tech socks for your race, maybe tie your shoe with a little more room in the toe-box and tape the nail down with a band-aid if it hurts? Oh and make sure ALL your toenails are trimmed. Good luck on your race!