I tend to drive a lot for fieldwork. I also have bad knees, partially because of genetics and partially because I developed repetitive stress injuries in high school and college. So the length of time I can hold out before stopping and taking breaks (and therefore, how long it takes to go somewhere) depends on how long my knees hold out.
Cruise control is a godsend. I last less than an hour without it, and I can go at least 4 hours (or until I run out of gas) using it. I put the car in cruise control as much as possible, even if it means just for a minute or two. I'm guessing that the first major thing to wear out in my car will be the cruise control system, if that's possible.
So, one of my pet peeves is getting stuck behind someone who doesn't use cruise control, even though I know they have it. I start creeping up behind someone, so I change lanes. That causes them to realize that there's another car about to pass them, so they do what? They speed up. I hate, hate, hate hanging out in the passing lane without passing someone (plus, it's illegal in most states), so I'll drop back. 30 seconds later, the other car slides back to their original speed. Usually I get fed up and accelerate way past all reason so that I make sure that I leave them behind.
I'm going to be driving about 8 hours (give or take 2 or 3, depending on the traffic gods) tomorrow to get to my parents' for thanksgiving. I have a feeling that my knees are not going to be happy tomorrow night.