I have to disagree with longrider42. An actual road bike is much more comfortable on longer rides than anything else. My first ragbrai I road a shwinn super sport flat bar road bike. That bike is a nice bike for general riding but to be honest is not a very good bike for ragbrai. It has 2 problems in my mind. The first being You only have the one hand position. the other problem is that you sit up to much and catch more wind.
A standard roadbike is by far the best choice. You can either buy new or used. If your price range is only about $550 then I would try to find a used one.