My comments on what's already been said:
I have limited experience with adding protein to the on-the-bike regimen. I bought a can of Accelerade and used it for various intensity rides. I never *noticed* any benefit. For shorter, intense efforts, I felt it was worse than a carb-only beverage. I also tried it in a plenty difficult timed century with ~10,000 ft of climbing. By the end of the event, my kidneys hurt. Never had that happen before, and I don't know if it's related. Thanks anyhow... I was hurting enough! I'll stick to carbs. For 12 hour events, I dunno. Try it, see what you think.

As far as prep, RWTD nailed it. Why would you do anything other than top up the stores? If it's prep for a special event, also taper and pay close attention to carbo loading, etc.

Finally, gotta disagree with Koffee on the pre-ride meal. If it's gonna be hard at all, I eat 2-3 hours before. BTW, this meal always includes protein, usually in the form of 2 eggs. In fact, it's not too different from my normal breakfast. (It's the meal the afternoon or evening the day before that makes the big diff for me.) If the event is a major objective, and particularly hard, I'll eat a powerbar 1 hr before the start, and an energy Gel 20 minutes before. It comes down to this... once you're putting out, about 300 calories per hour is all yer gonna get. Choose 'em wisely.