I don't ride a beater but I'm lucky enough to be abel to bring my bike into my office with me. Is there any possibility that you can bring your bike inside with you? If not, you could build-up a pseudo-beater like others have suggested so your commuter is less arractive to potential theives.

Try using MapQuest or Street & Trips or some similar software to plan a bike route. If I took the same route on my bike that I take in my car (I drive on occasion too) it would be about a 12 mile ride on some very busy roads. My bike route is 14 miles and for the most part it's on much less busy residential streets. Don't be afraid of riding on the street as opposed to the sidewalk. In many states it's illegal to ride on the sidewalk. It happens to be legal to ride on the sidewalk where I live but I'd rather not deal with dodging slower riders, people wlaking their dogs, little kids, etc... It's not illegal to ride on the street in any state (that I know of) and that's really where we cyclists belong in my opinion.