Butterflies and excitement

I don't mean to sound repetitive but it's tough to have my mind on anything but New York right now. I did my last 20 mile run yesterday and it was rough. In theatre terms that would be a good sign for a great openning but this isn't theatre. Ever since Chicago's marathon last year, I've been hooked, but lately running has been tougher. I hope to meet my goal but am nervous of not having it in me. Overall, I just want to enjoy it. I can't seem to shake the butterflies. Any advice?