I agree with most everything said so far- but assuming you don't have unlimited time for a lot of really long rides, try to do a lot of 2 to 3 hour rides, mostly in zone 2 as others have said. Add a longer ride every couple weeks, as best you can. This has worked for me to prepare for multi-day rides averaging 60+ miles/day.

I agree that really long slow rides alone can get pretty boring.