I have never ventured much past 50 miles. It usually takes me about 4 hours. When I use my GPS my average speed is typically 17-18 MPH. That is on pretty even terrain without a lot of big climbs. Of course if there are any mechanical issues or flats, that adds time.
As it usually goes my dreams do not come close to reality. I cannot ride every day, nor do I have 3-4 hours a day to train properly. So here is the current plan to survive a 100 mile ride.
1. Ride as much as possible.
2. Get a good set of tires (avoiding flats=finishing in less than 8 hours).
3. Drink a bunch of water.
4. Shove my jersey pockets as full of Jelly Belly Caffeinated Sports Beans and Cliff Bars as possible.
It is not much of a plan. We will see how it works out. My goal is sub 8 hours.