I'm still on 65", same as always. That's good for me up to about 65-70 miles at 75ft ascent per mile. I might not manage another 30-35 miles, unless it was a lot flatter (in which case I'd be wanting a bigger gear).
Been mulling over this and I am fairly sure you would manage the ride. I'm not really into the numbers game except to give a distance on "Your ride today", so your 75ft of ascent per mile means nothing to me. However, I do know that when a group of people on fixed are riding together it is so different to anything else I have experienced.
I have ridden on a few occasions on fixed with several of those on this thread, one of whom is well into his sixties and I am confident we will all make it.
Rides of 50 - 70 miles [depending on terrain] are good preparation because over 100 miles the group will do the rest. I have no idea of your experience or anything else but if it was me I would try a rear sprocket with one less tooth and ride maybe 68" or 69" and see how that feels.
The 8 hours is not written in stone, and anyone struggling a bit towards the end will get a lot of help from the others. In my experience you are less likely to encounter testosterone fuelled / erratic riding with those on fixed because we have absolutely nothing to prove.