I know I might take some heat here, seeing as this is the road bike forum, but I'm going to recommend you find a cheap single speed instead of a true road bike. They're dirt cheap, and there are loads of them on Craigslist for as low as $100 built up from 70s and 80s frames. No gears means very little maintenance, tuning, etc. and also means you don't have to know much about bikes to buy one used. What you see is what you get. Components grade, quality, brand, etc is less important this way. Find one, check it out, make sure it's a comfortable fit, and ride it. If the tires spin smooth and straight and everything seems to work as it should (i.e. propels you in a forward motion), you're good. It will build up leg and lung strength, and you might find it's very relaxing to not have to think about what gear you're in. Nice for riding around town with friends.

Bikesdirect.com has a ton of new ones too as cheap as $250 shipped.
This one would probably work well. It comes with brakes (some of them don't), steel, relaxed geometry frame for more comfort, and decent entry level wheels.

http://www.bikesdirect.com/products/...steel.htm#size