I am close to 75 and do not do well with long continuous hours driving. Even when I was much younger I could not handle more than about 10-12 hours of driving which would total maybe 500-700 miles.
I do make long cross country trips. I live on the East coast and mainly visit the West and Rocky Mountain areas. I found way to handle those long drives without being stressed and tired. I drive 3-4 hours and then stop for 1/2-1 hr. In addition to fueling if necessary, I get into the truck camper. Have something to eat and drink and take a power nap for at least 20 minutes. I am then refreshed and ready for another 3-4 hour leg. I don't like to drive at night, but in the Summer with long days, I can drive for a total of about 10-12 hours averaging 60 mph; i.e., 600-720 miles. Typically I am towards the bottom end of that range which means I have about 3 days to get past flyover country and the marathon is over.