At 71 now that is a full day for me. In fact on a long trip we plan no more than 300 a day. Not too hard on interstate but plenty for us on secondary roads. 

Many years ago I figured 600 a day max miles and that wore me out. 450-500 on very good roads was a lot more comfortable for traveling cross country. But as I have aged I no longer want to "work" at it. We take it easy, stop often and enjoy the ride. Went from Kansas to Seattle, Wa. to visit friends and go on a cruise earlier this year (90% on secondary roads). Think it was 7 or 8 days to do it. Longer coming back home via different route.