I don't think they would only visit cities and ignore rural areas/small towns. Events can only hold so many people. If a candidate visits LA it's not like they can shake millions of a hands in a single day.
I think they would visit more places than they do currently because there are thousands of smaller cities/towns where they could still get huge crowds and reach a lot of voters. I'd fully expect to see candidates start in one state and go up and down and across the country visiting thousands of different places.