+1 on previous responses.
I think in terms of time more than miles -- at my typical average speed, how long will the ride last and how much time between rest stops. I generally plan for 1 water bottle per hour (more on very hot days, less in the winter) and usually have two bottles on my bike -- one with plain water and one with Gatorade or bottled iced tea with sugar so I have some variety.
For food, I usually stop for a snack for rides lasting more than an hour. For rides that are 2 hours or so, I will just have a cereal bar or fig bar, 100-200 calories. For longer rides I have a variety of snacks with me, including Gu, Jelly Belly sports beans, cereal or fig bars, crackers, and/or Honey Stinger waffles, and will eat 200-400 calories each time I stop, which is usually every 60-90 minutes. The common denominator is that everything is easy for me to digest, mostly carbs (since fat and protein take longer to digest) and not super sweet. This is what works for my palate and stomach. If I have a sandwich, it will be just jelly on bread or a tomato sandwich. A little fresh fruit is also nice, especially grapes. (And peaches if you're in ever in the Frederick MD-Gettysburg PA area in early September. Best fruit I have ever had. )
I have friends who bring pb&j sandwiches on every ride and that's all they eat. I've seen other friends stop mid-ride and eat egg sandwiches, pulled pork, cheesesteaks, all sorts of things that for me would be too heavy.
It mostly comes down to trial and error while you do your training rides to prepare for the event, figuring out what works for you. Also experiment with what you have for breakfast before the ride, and pay attention to what you eat on the days leading up to the event. I've learned the hard way to avoid big heavy meals the night before a big bike ride, and to stick with foods that I know won't be a problem for my somewhat-delicate digestive system.
Good luck!!!!
- Gray 2010 carbon WSD road bike, Rivet Independence saddle
- Red hardtail 26" aluminum mountain bike, Bontrager Evoke WSD saddle
- Royal blue 2018 aluminum gravel bike, Rivet Pearl saddle
Gone but not forgotten:
- Silver 2003 aluminum road bike
- Two awesome worn out Juliana saddles