agree with gym addict on that one, you dont need to change your diet massivly weeks in advanced, just get the training in, night before have plenty of carbs for arguments sake, and on the day, SIS go gels, one before you go and one 7-8 miles in should do the trick, as well as the usual advice about drinking loads etc there isnt too much more to it!
Dont worry too much about the calories on the day, remember during your run you will probably burn more then your reccomended daily allowence, so consuming a few calories during the run wont hurt, obviously if you are trying to loose weight, then dont consume too much during training, but on a race, you want to get the best out of your run, so do whatever (as long as its in the rules and legal) helps!