It may honestly be easier than you think on a comfortable bike. My wife and I did an 89 mile ride together and I was on a bone stock Trek 3700 mountain bike, not an ideal choice for a long ride. She was on a Schwinn Frontier Sport X Mountain bike. The longest ride we did before that was 2 months prior and was 26 miles. We were averaging riding between 3 and 8 miles per day on local rails to trails paths just to unwind after work for about 3 months before our big ride.
Build up your length of ride slowly to let you body get used to it and you will do your 100 with no problems. Especially if you are on a confortable and much lighter bike than either of us were riding.
Oh, and by no means were or are we the picture of health. We were smoking a pack and a half a day each and hungover from the night before when we started the ride.
