I am retired and a full timer. I enjoy driving. I absolutely love traveling down the road from one place to another.
You said you're retired. So you no longer need to be in a hurry. When I retired and started fulltiming I needed to get into a different mindset. When I was employed and only had a couple weeks for a vacation, I always needed to make big miles going and coming. Now things are different. I don't have to rush now - ever ! I don't even wear a watch !
I try to limit my day to about 250 miles, but 200 is fine too.
I usually run 58-60 and stay in the right hand lane. At that speed I don't come up on too many people so I don't need to pass many. I also have a bit more reaction time in the bank and can stop a little quicker. On 2-lane roads, I always pull over if traffic backs up. I spent about 2 1/2 months wandering around California last summer and as I recall, California is very good about having frequent turnouts on their 2 lane highways.
So my advice is to run at a speed where you are relaxed and comfortable. And remember that you no longer have a schedule.