I think the appropriate length of a drive is rather dependent on the types of road that make up the route.
For a day's tour around twisty B roads, 300 miles is asking a bit much, I think. Fine for a Motorway blast for a few hours to get somewhere, but personally I like playing about in the countryside. 3 hours in the morning, lunch, 3 hours in the afternoon averaging say 40mph gives not much more than a couple of hundred miles.
