Don;t worry about pace, it's utterly at the speed that feels right at the time, you can always find someone or a group that's doing a comfortable pace
Last year I rode quite quick some of the time and plodded along for others as I felt more or less tired,it's a long way and overnight too so it's hardly a race, I was surprised at how low my average speed was but there's a lot of stopping at junctions and I might do a pub or two this time, Dunwich ain't going anywhere
Anyway, it's quite hard to stay with someone in the dark, I think me and Will managed it for about 50 odd miles and got separated thereafter, one bike in the dark looks much like another, especially when you have a few dozen to chose from, I think we got separated a few times and happened upon each other before getting finally parted, Will sacking out for two hours at the stop put the final kibosh on it
I did some long stretches in the wee hours on my tod that weren't much fun and some long stretches approaching dawn with someone that was brill, a right good chat and ride at a good pace, he had wine gums and I had chocolate raisins, unless you're bloody good at map reading you tend to wait at tricky junctions and wait for a larger and better equipped group to catch up anyway, the amount of groups going the wrong way after discussions was quite alarming, people with gps tend to get followed although they're nothing like failsafe at all
a big group went left up Clapton Road after two miles, that was in London with daylight and signposts ffs
relax, it'll be fine, last year I'd done two 50 miles rides in the months leading up to it, both easy ones, otherwise commuting, it was a strain but it was never in doubt, you just keep going, especially when there's girls and people on MTBs around