I would not overthink it. Going at 15 mph one covers 100 miles in less than 7 hours which is not that difficult. Riding one hundred miles the following day is a different matter. My first 100 plus mile ride was when I was a teenager living in Altadena and I bicycled to Santa Barbara to visit a former neighbor who had moved there with his mother. Road back home the next day and never thought much about it.
It is a good idea to stay hydrated and to bring snacks you can eat during the ride. Two water bottles will be more than enough if you start the ride well hydrated. If you get dehydrated your blood thickens and your heart works much harder and heat stroke is a potential problem. Around that time I road 100 miles to visit friends in Cabazon and I came very close to having heat stroke as the heated pavement was raising my core temp to a much greater degree. Being on the pavement during the peak heat load period was very unhealthy as I realized later. A local Dallas marathon runner died during the Olympic Trials when he experienced heat stroke.
I love raisins as a snack as the sugar goes quickly into my blood stream and I can eat them one handed. I use a small handlebar bag for a few small snack size boxes of raisins. Add a bag of nuts to munch on and your will be in good shape. I have found a high carb breakfast of pancakes fuels me for long days of riding. Pasta the night before for dinner (carb loading) also can help and was a common practice for pro bike racers before the advent of snack bars that the riders are paid to promote.