Having a Plan B is majorly good advice.
I well and truly bonked at Boston because I was too nauseous to eat the gels and protein paste I'd brought, and by the time my brain started going south, I didn't have the sense to just take the Gatorade from the fluid stations, since I had it in my barely-functioning head that I have an issue with Gatorade. (I do have an issue with any fructose, natural fruit or HFCS or anything in between, but it doesn't manifest itself until AFTER my run, so for a goal race, bypassing it was the wrong decision.)
Moral of the story, drink the Kool-Aid and everything will be fine.