A century, or 100 mile ride, is a big goal for many cyclists. I've been riding recreationally since I was a teenager, and I was 28 before I did over 100 miles in one day. Since then I've done quite a few of them, including a double century, so I've pretty much gotten to the point of considering a century just a long ride. So here's some advice for people who want to do their first one.
The most important thing to completing a century is determination. If you want to do it and think you can, you will. It may be painful, but you'll finish. If you don't think you can do it you'll find some excuse to drop out: a cramp in your calf, rain, whetever. The rest of this article is all about making it hurt less, but this is all secondary to the right attitude.
What sort of bike should you use?
To get your body ready for a century, you need to ride. This may seem obvious, but that's really all you have to do. Once you've gotten to the point where you can do 70 mile rides without knocking yourself out, you're ready for the century. Of course, you have to do your training in terrain that's at least as difficult as where you'll be doing your century. If you find you can ride a flat 70 miles easily and then try to do a century in the mountains, you may find is a very unpleasant experience.
When I was doing centuries regularly, my training schedule was: Tuesdays and Thursdays, ride 30 to 35 miles thru' hills, then a longer ride (40 miles or more) on the weekend. Now I have a full time job and don't spend so much time on the bike. I still aim to ride 75 miles a week. I get 25 miles/week by commuting, do one evening ride that's 20 to 30 miles long, and then do a weekend ride of 30-50 miles or sometimes more. I can still do centuries, but they're not as easy as when I was riding more!
What to take with you, and the ride itself
In my opinion, the best bike for a century is one that fits you very well and is comfortable for long distance riding. Lots of people ride with their seats too high or too low, or with too long or too short a stem, things like that. There's a variety of advice on the web. The best in my opinion is written by Peter White and Sheldon Brown. If you don't feel comfortable setting up your bike with these guides, go to a bike shop.
People often debate the pros and cons of different kinds of bike for long-distance riding. Some people like to go fast and will ride their racing bikes on centuries. Others just put slick tires on ther mountain bike to give it a go. Both approaches are fine, it all depends on what your goals are (to go really fast, or just to finish!).
If you have a mountain bike and would like to try a century, here are some tips. First, put slick tires on your bike. I find that 1.25" to 1.5" tires work well for me. They're wide enough to give some cushioning, but narrow enough to be reasonably light. Second, get some bar ends if you don't have some already. These allow you to have a change of hand position and let you stretch out a bit. Third, if you can, put on some narrower handlebars. If you use your bike off-road, you probably have quite wide bars. These are great on trails to help you negotiate those sharp turns, but on-road they just increase your wind resistance. This may not seem like a big thing, but the longer the ride is, the most important it becomes. On a century, you'll be going more or less into the wind for about 50 miles! If you reduce the width of the bars you'll bring your hands and arms closer together and this will reduce your wind resistance somewhat. The easiest way to do this is cut a bit off the ends of the bars. However be cautious: you can go too far. If your bars are too narrow, the bike responds too quickly to small hand movements and it feels unstable. I find that flat bars about 21" wide work for me.
Preparing your bike and gear
Tools are important. Always have a spare tube (or two), a patch kit, tire levers, and a pump with you. It also helps to have Allen wrenches with you (one for each bolt size on your bike) and a small adjustable wrench and some screwdrivers. You don't need separate tools for all of this: I've seen one took that combined all these things, along with a chain tool! (Some people suggest carrying a chain tool, but I've never needed one. I guess I'm not strong enough to break a chain.) I have a very neat tool called a Leatherman. It has pliers (which are useful for pulling thorns and pieces of metal out of tires), several types of screwdriver, a file, a knife, and a can and bottle opener.
Sunscreen is important, since you'll be out under the sun for many hours. You'll probably sweat a fair bit, so it's a good idea to take a little bottle with you to reapply when you're at a food stop. Also, if you don't wear a helmet, a hat is very useful for keeping the sun off your head.
For information on food and drink, check out Bike Food.
If you are on an organized ride, try not to stop for too long, maybe 10 or 15 minutes max. Otherwise your legs cool off, and it really hurts when you get them going again. You need to get off every now and then to stretch your legs, but just long enough to fill your water bottles and go to the bathroom will be sufficient. If you're tired, just go slower. You'll give your legs a change to rest, but they won't cool off and you'll still be making progress towards your goal.
When food stops aren't provided for you, you either have to bring it all with you, or stop to buy it. If you bring it with you you can avoid long stops by constantly snacking, but if you're buying, it can be hard to find enough sources of food for frequent snacking. Thus if you're doing the ride on your own or with a few other people, you may want to stop for a big lunch. Your legs will cool off, but it can help refresh you for the remainder of the ride ahead.
More bike articles
Don't get lots of new parts the day before the long ride. Go over your bike the day before, checking all the screws to make sure they're tight. My toe clip almost fell off once due to a couple of loose screws. The thing is that loose screws often work just fine. That is, until they fall out. So you can't just assume that because your bike has been working fine all this time, it'll continue to work fine on the long ride.
Do your packing the night before. Load up your bike bags with whatever you'll need, put your water bottles, filled, on your bike (some people like to keep them in the fridge, but what's the use -- they'll warm up fast enough anyway, and there's the risk that you'll forget then if you drive to the starting point of the long ride), attach your pump, reset your bike computer, lay your biking clothes out with sunglasses and helmet nearby. If you're going to get a really early start, make up your morning tea or coffee the night before, and nuke in the morning to heat it up. Maybe it won't taste so good, but the less you have to worry about the day of the big ride, the happier you'll be.
Now, this sounds like alot of stuff to do, but the thing is that then you'll be very well prepared, and the your 100-miler will go seamlessly.
Best of luck!