I'll add that it seems to be that the more of this stuff you do, dare I say that the easier it gets. I remember being fairly well trashed at the end of my first 400K and 600K. I've done a fair number of those distances since, plus some 1200Ks, and I feel better at the end of every one of them. The rides are still plenty challenging and require a good base of fitness and some hard work, but my body isn't in rebellion at the end of them like it used to be. I think part of this is that I learned to ride "smarter" over the years -- pacing yourself on rides of these lengths is critical -- but I also think the body just gets used to the familiar "routine" of doing a long ride.
As for the enjoyment of it, I've had plenty of tough moments on the bike but I don't think I've ever had a boring one. I'm easily amused.