We are starting to plan a summer road trip. We have purchased the Road Trip USA book and note that there is a complete absence of time scales. We will have 4 weeks. We do not want to have to spend every day in the car and we would rather travel on "two lane black top". Any advice that seasoned travelers can give about the kind of time scales involved in going east west (or vice versa) or top to bottom, how much driving in a day, days off etc would be most gratefully received.