I don't get saddle sore generally for the first couple of hours, and then it starts creeping in. DD took Bill and I, 8.5 hours last year. So my @rse only hurt for the last 6.5 hours really. The last couple of hours were ungodly. I don't know how you prepare yourself for that sort of time in the saddle, short of doing the Tour de France. I did liberally apply Bum Butter, not sure if it helped, or if the slimy feeling in my pants was just a pain distraction. From what Bill said last year, I am half expecting him to turn up with a motorbike seat with a pillow gaffer taped to the top!
The problem is as you get more tired you spend more time rooted to the saddle. With discipline ( and a little will power) one can take every slight incline or small rise to drop down the block and rise from the seat.. maybe time it with a jelly baby or sip from your bidon? Don't wait till it's aching before starting or chances are.... it's already too late.
Then the mind games begin..... tick tock - miles, bum...