Also remember that in order to ride 100 miles once every few months, you don't have to ride 100 miles all the time. Ride hard for 20, 30, 40 miles. Once you do the century and drop your pace a bit, your body will have no issues with the longer distance. When I rode my first century the longest training ride I had done was 53 miles. They were hard and fast miles, but still.
I guess my point is, don't kill yourself worrying about distance as much as focusing on building strength and endurance.