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Oh yeah, I'm aware of this. I did south downs double with the wrong saddle, 26 hours, and large arse blisters later, I've changed saddles, and never ride with cream now, and it's all turning leathery! But even with that, my mate who did it this year had bad blistering. Every night, wipe down with baby wipes, sleep with shorts down around ankles if you're camping out, and let it breathe. Lots of sudocream. In my mate's words "It only hurts in the morning, until the blisters burst".
Ew.
Time in the saddle beforehand as well as the best cleanliness you can manage during the ride will help avoid arse issues but with that much off road you're gonna have some unwanted anal pounding ;)