My tours have usually started with either a route from Adventure Cycling or one recommended by someone else. That said, I have typically just winged large sections of the route once under way. Sometimes that might just mean deviating for fifty to a hundred miles or so at a time and sometimes it meant leaving the route for most to all of a large state.

I like the way that has worked out for me. It allowed for plenty of flexibility, a good idea of how long/far the ride would be, and no need to do a lot of pre planning. I don't see why just winging the whole thing wouldn't be fine too though.