IMO, consistant daily commuting is one of the best preps for a century you can do. Whether you realize it or not, you're building a great aerobic base. Start trying some longer rides on week-ends... you'll be surprised. So, given that, if you're just trying to complete a century, and not set a ground speed record, you're well on your way. Eat ~300 calories/hour, hydrate sensibly (that's covered frequently in the T&N forum, magazines, etc.), and enjoy it!
If you're doing a supported century, there will be rest stops about every 20 miles. That's the point, you're paying for support, fill your water bottles at the start and refill at each stop. Otherwise, carry basic repair tools and use the food provided at rest stops. Put some extra in your jersey pockets to be sure you're set between rest stops.
If you're doing a self-supported century, pick a route with stores on the way. You want food that has easy to digest carbs. In general, that means low fat, low protein stuff. Fig bars, bananas, nutri-grain bars, pretzels, raisins, GU, etc. fit the bill. Load your jersey pockets, and avail yourself of the stores along the way. Many will let you fill water bottles, and it's a good idea to have at least one bottle with Gatorade, sweetened ice tea, or whatever.
BTW, if you're doing an unsupported century, one of the things you're "practicing" on the longer rides is the logistics of eating and drinking for a century. Try to make time to build to longer rides on week-ends... it's important.