Your drivetrain is of most immediate concern. Clean the chain thoroughly to remove grit, let it dry, and relube. Lots of ways to do this, both on or off the bike. Clean the cassette. Clean and lube the derailleur pivot points and jockey wheels. Wipe grit off the chainrings. Wipe the brakes clean and lube the pivot points. This article will be helpful: http://www.teamestrogen.com/articles/asa_happy.asp

Bearings are another concern. A century in the rain might merit a complete repack, though most people can go a year between repacks if they don't ride in the rain much. First check to see if your hubs, bottom bracket, and headset have sealed bearings or servicable bearings. You might get this info on the manufacturer's site or you can tell by looking. Sealed bearing should be fine for several years, but feel them every once in awhile to see if they are still smooth and fairly friction-free. If you have servicable bearings and want to do the job yourself, you'll need cone wrenches for the hubs, and a crank puller, lock ring tool, and pin spanner for most bottom brackets, plus good quality grease. Take your bike to the LBS to make sure you get the right tools. You'd want someone to show you how to do this the first time, or read the ParkTool website carefully. A good bearing adjustment is based on feel, plus some trial-and-error.