Unfortunately, your question cannot be well answered unless by someone of the same age and has a similar rig.
The real answer, is start out planning 6 road hours a day and then modify your plan. Hours is actually a better gauge than miles. Hours make you weary. If your ride is good, you may be capable of more, but always remember that driving when you are tired is a very easy way to end up dead. Most likely that 6 road hours will translate to 8 or 9 clock hours and that is plenty.
Wife and I co-drive a very comfortable coach, and we have done some long pulls. We now reserve those for times when we have little choice. They are never fun.
Unfortunately (again) is the fact that both Strips and Treets and Street Alas are gone. These where the best planning things out there and they were not internet dependent so the navigator could update the plan while underway.
Matt