I would also recommend trying something different nutrition wise. It sounds like you should have a good enough level of fitness to ride 100 miles you just need to make sure to take in enough calories and liquid. I need about 300 calories per hour to maintain for long rides so a cliff bar and sports drink works great but luckily my stomach isn't picky and I can eat just about anything on a long ride. I also make sure I am drinking something with electrolytes as well, one thing I really like to add to my water is from
elete. It basically gives you the electrolytes of a sports drink but without any flavor or sweeteners, my water still tastes like water which makes it great for mountain biking when cleaning out a camel back is a pain.