@
blowboat, have you done day-long rides (century or similar) before? If so, was your saddle been comfortable on those rides?
I'm having trouble untangling possible effects of long days in the saddle from consecutive days in the saddle. If you have similar symptoms (though not as extreme) after 8 hours in the saddle, you've either got the wrong saddle or you need to ride more to toughen up before the big ride. If it's just consecutive days, well, start with the chamois cream the very first day; otherwise, you'll damage the skin and it won't have a chance to recover.
I will say that it's much easier to vary your mileage if it's not an event ride or supported ride; you simply say, my butt is sore, I'm going to camp or get a motel after 30-40 miles today instead of pushing to 75 miles.