scratch, "what is your average for the 100 miles"???
When I did an 80 over some big hills approx 3 weeks ago I took 2 large bottles with SIS (energy drink) if you don't know, 2 bananas (early food) to be eaten within the first 20/30 miles, then I eat approx 4/5 snack bars, I currenty using Jordans "Frusli Red Berries" and I was fine, in fact we were sprinting for the last town sign and I won (get in), got home with an avarage of 19.5 so I has happy and felf OK TBH.
As other people say eat little and often and drink drink drink, ever stop at the local shop and buy a coke or somethink if you run out.
However I must say WELL DONE for the 100 miler, I've not done that for a few years.