Roger,
There are TWO sides of opinion re this matter re mileage and driving time and each are equally valid.
Personally my main concern would be the time spent "on the road driving" rather than actually enjoying the various areas you will, by reason of the itinerary and distances involved in the available time, be forced to drive through rather than stopping to explore.
Your plan does seem to be, to my mind, very much "A Busmans holiday" Insomuch as your holiday will be pretty much the same as your working day, i.e. driving for a large proportion of it.
My working life was spent driving, and thats the very last thing I wanted to do on holiday.
Either way I hope you have a safe and enjoyable trip, but I am sure you can understand for many it isnt what they would class as a "holiday" But each to his own of course.