Driving is more of a mental fortitude feat than it is physical. For me, the mental drain from the mostly mind numbing uneventful surroundings is awful. You need to stay alert, in case something happens. And in a year of driving on average 100-150 miles a day, only about 10 times that I can count did I encounter something unexpected.  If I’m a passenger, I don’t really care how long it is. Just stop every 3 hours or so because I need to stretch.