I've done a 600 mile day on the Glee, and a few on the Wee, one outlier at 750 miles which I don't intend to repeat without an emergency scheduling constraint (which it was, had to hightail it home). The Glee's seat is more grippy so you have to treat it differently after a while. On any bike, I generally take a regular quick break with stretching after 60-90 minutes. Usually I match that up to gas or food stops, but of course sometimes the scenery has to be appreciated for a while. I plan trips so that they have legs that fit a decent break schedule, and try to anticipate varying times at which I reach exhaustion.
I usually find that such days are actually easy to plan. What I find difficult is how to plan a series of them. I find that I can do one 600-mile day, one 550-mile day, and then one more day that depends entirely on the nature of the trip and weather, and then any fourth day is essentially useless and I need to take that day off or at least not ride a long distance. So that essentially means I can go 1600 miles in some direction for three days and then take a day off, then go in some other direction a similar amount.