If it makes you feel any better my calves are terrible at the moment and I'm running at London too. Anything over about 14 miles and I'm having problems, then the next time I run they've not recovered. I think on the day the adrenaline and atmosphere will keep you going.
Hi Melanieb, Please tell us how you did! Were you doing one of Eric's plans? I had a friend who started training for a half marathon this January, and kept straining her IT Band...she started running comfortably 4-5 miles three times a week, but when she wasn't running with me on weekends, she would pop up to 10 miles. Even though she was foam rolling every day, she still wound up having IT Band pain. It subsided enough so that she was able to run the half, and ended up with such IT Band pain at the end that she wound up not being able to run for a couple of months...she is just starting back. Muscles fatigue the longer you run past your limit, until running in a compromised state leads to injury, because you are repetitively stressing the muscles in a fatigued state, which isn't' like running with muscles at the beginning of a run.