Yes, I've done it. You are right to be worried about ass complications. Best advice: get a gel saddle (or a gel saddle cover at WalMart if you need to save the money -- the ones with the cutout down the middle). And get some gel shorts if you can, or sew some gel into some riding shorts.

The double gel will go a long way toward preventing problems. You don't really have time to wait for the toughening-up process to be effective.

Avoid seams in pressure areas. Stay clean. Stand up periodically to relieve the pressure on the soft tissues, and provide some circulation and oxygenation to the tissues you are sitting on (this avoids tissue ischemia problems). You should be all right.

Also, don't pressure yourself and strain at keeping your speed up. That approach is often exhausting. Instead, pace yourself. It isn't a race. Just find a reasonable, sustainable pace rather than wearing yourself out.

It's a lot better to do eight or ten or even twelve hours of riding at a reasonable pace than five hours of riding at an exhausting pace.

Giving yourself some extra time each day and overall (to attain the mileage you are after) makes sense. It takes some of the pressure off. Having the extra time is a good feeling.

Also, getting high quality rest and sleep is important for a good state of mind, physical recovery, and full enjoyment of the trip.