Like many here we drive by the number of hours since we are retired. If we have to be somewhere by a certain date, we leave earlier rather than busting our butts to get there. Five to six hours a day is about right for us and the dogs.
Yes, in our younger years we could drive forever. When we were first married, we would call in "sick" on Friday morning after having left Dallas on Thursday after work for Estes Park. We would drive all night arriving Friday morning. We would sleep a few hours and hike Friday afternoon, Saturday and Sunday morning. Then we would drive straight through to Dallas and be at work Monday. Just writing about it makes me tired.