You can do this! The secret is to eat enough as you are riding. Start eating early and eat little bits as you go along. What you were probably feeling at 70 miles was the beginning of the "bonk" with low blood sugar levels. Unfortunately I cannot tell you what you should eat as eveyrone is different. What works for rider "A" will make rider "B" ill.

I find that I can generally do 50 to 60 mile rides at a decent effort without eating, but anything longer than that requires that I eat while riding. You should experiment and find out what works for you. Clif Bars and peanut butter and jelly sandwiches are my long ride staples, but others will probably chime in and tell you that these will make you as sick as a dog. You really need to find out what works best for you. Don't wait until the 100 mile day to figure this out!