www.bikejournal.com

+1 on what's already been said... YOu don't have to work up to 100 miles all the way... marathoners don't practice by running 26 miles I b'lieve a general rule of thumb (though humans have a fair amount of variation) is that for a special event wtih food and rest stops you can go 3 times as far as what you could go on a normal day - so when you get to the point where a 35 mile ride doesnt' leave you tired out the next day, you're probably ready to taper down and do a 100 mile ride. Or you might find that you want to do 35 miles every day

(unlike marathons, you don't need the same kind of recovery time, either. Cycling is a lot easier on the bod.)