Road bikes are meant to be rode for long distances. If that is your goal, and you don’t know how to choose a bike, do not buy a used bike from somebody off Craigslist. (You can definitely get some good deals there, but if you don’t know what to look for you can purchase a lemon quite easily.)
I’ll offer you a few suggestions. First, if you do go and check out some Craigslist bikes check, first off, for rust. If the chainwheel, cassette, or derailleurs are rusted, not a chance. Means either the owner did not take care of their bike or it has been sitting around for far too long. Second, if the parts look new or well maintained, check the frame out. Look closely, any crack, however tiny, and you walk away. And finally, take any bike for a test drive before you purchase. Don’t just take it around the block, go for a good half hour or so. Make sure the bike feels good, you’re comfortable on it, and everything works on it. (If you can’t tune up the bike yourself then don’t buy a bike that isn’t properly tuned up.)
The preferred suggestion, if you want a cheaper bike, is to just find a local bike dealer who buys used bikes and then fixes them up himself. Just go to a local bike shop or two and ask the employees if they know any shops like that. Local bike shops, not big commercial shops, are glad to help people out with their bike questions. For the price of something on Craigslist you can get a properly refurbished bike for a few dollars more (or less, depending).
Last suggestion is, if you’re at all interested in a new bike, a Surly Long Haul Trucker. I purchased one the other month and I’m still in love with it. She is going to take me across Canada next summer. The LHT is a nice ride which comes with good standard parts on it and it isn’t too expensive. On review, if you’re just riding casually around town, you don’t need a LHT. Go with my second suggestion. You’ll get a decent bike at a decent price that is tuned up and ready to ride.
Goodluck with the bike hunt, whatever you choose.
Oh, and you’ll probably be around a 56. It depends, however, on the build of the bike.