200 miles in one day? Good luck with that. How is your bike fit and cadence? You will really get to know the little things that are off on an all day ride. For me the only thing to prep for long ride is to do long rides. I commute , mt bike and tour. All help. For a 100 miler, you should at least get an 80 miler in sometime beforehand. Pay attention to hydration and fuel. I like real food like sandwiches and bananas as well as bars and drinks.