Been there, done that, got the t-shirt (and had a blast).
That was 7 MS150's ago for me, and I ended up enjoying cycling so much that I've just joined a race team last year.
If you're like I was, you're more concerned with your ability to ride 100 miles in a day than any care about how fast you finish. The key there is to start acquiring some time in the saddle. Don't worry about how far you ride at first, just get on the bike 3-4 times a week for at least 30-min and start getting your butt, back and neck accustomed to the riding position. Get a pair of biking shorts immediately! Concentrate on keeping your upper body relaxed, changing hand positions periodically, and spinning along without mashing hard on the pedals. Many beginners will try to start out in too hard a gear, which will lead to muscle (and possibly tendon or joint) soreness. It shouldn't feel hard to push the pedals around, after all you're going to have to do it for an entire day. As you're initially getting accustomed to riding, you'll also want to listen to your body and look for any acute pain which could mean a bicycle that is not properly adjusted to you. If this happens (anywhere but in your butt), then you might want to have a local bike store or an experienced rider check the fit of your bike and your riding position. Will you be riding a road bike, mountain bike, or hybrid?
After your are able to comfortably ride 3-4 times a week, then you can start increasing the mileage on 1 or 2 of those rides. Try increasing the mileage of your longer rides by about 10% per week. Make sure you are taking water with you on these longer rides (and food for rides longer than 1.5 hours). About 1 bottle per hour is a good thumbrule in warm weather, and I'd say a energy bar per ~1.5-2 hours. Get used to eating and drinking during your rides, because if you get dehydrated or bonk, it's very hard to recover. The MS150 will pretty much be a series of 10-mile rides, so feel free to take short breaks during the longer rides if necessary. It might be a good idea to keep a training journal where you record the time, mileage, speed, etc. on your training rides. This will help you appreciate your progress and keep you motivated. There are also organized MS150 training rides during the months before the ride, and I encourage you to join those and talk to others in your area who have ridden before or are newbies like yourself. Your local MS150 chapter will have information on the where's and when's. When you can get to the point that you can ride 60 miles at one time in training, you'll be able to do 100 on the day of the ride. At that point, just keep up your training routine until the day of the event, and have fun.
Let us know if you have any specific questions, and tell us how it went. Good luck!
