I have several friends and colleagues who are competitive cyclists, and according to the people who train them, two hours on a trainer are equivalent to 3 hours on a road bike. So... what Thorn said.

Many of them do a good deal of training during the week indoors, and then go outside for long rides on the weekends. When you use a trainer, you have complete control of your workout. You put the "climbs" and the "flats" exactly where you need them, not where they happen to be situated on the road.

Also, I wouldn't worry about being on a different position on the spin bike. It's not *that* different, and a big part of training is simply improving your cardiovascular capacity and your stamina. Also, you can adjust the spin bike to better mimic your normal riding position, if you like, for example, by bringing the handlebars down to the level (or a few cm's below) of the saddle.

My only concern would be that you need to know what it feels like to ride 100 miles, and I'm guessing you don't take any spinning classes that last 5-8 hours, so try to at least get some 80 mile rides in there during April, and if possible, do 100 miles just once -- like one or two weekends before your event.

Good luck, and congratulations on undertaking the challenge.

~Cari