Nancy

don't worry about the weight - get out to your local bike shop and try a few bikes, a hybrid, a cross and a road is a good place to start. a comfort or mountain bike will (possibly) be slower than you want. The best place to start is with a test ride. bring your helmet and comfortable clothes. don't buy based on color or brand, buy what feels good. the better it feels (and ask for the bike to be fit to you) the better chance you will actually ride!
180 lbs and on a road bike...