+1 about boring highways. try to blend the ride with back roads to. Helps to keep you awake.I've done a number of 700 mile days, a few of them all secondary roads (no slab)
Doing it on highway isn't too hard, just boring.
I would recommend taking a short break every hour or so, not every 150 miles.
If using the stock seat, go buy an Airhawk pad or at least a sheepskin cover.
Have at it!