For my double century last year, I bought the biggest water bottles that would fit on my bike (800ml and 600ml, but I'm 5'2 with a 46cm frame). I had a to peak top loader to keep a couple of sandwiches, phone, and battery packs in, small saddle bag for tools, electrolytes, and spares, and shoved my pockets full of food, bars, and a couple of gels. We had a support car with more water and food, but we also stopped at petrol stations and cafes along the way to refill water and food supplies if needed. By the time we passed the 150 mile mark, none of us were particularly hungry, but still kept drinking loads of fluid. I drank 15-20 litres of fluids easily that day.