good shorts/comfy saddle/chamois cream. maybe a slightly easier gear.
done the dunwich dynamo (125 miles) a couple of times fixed. dropped down to 68inches from 72 for a slightly easier time.
I’d have gone the other way TBH, higher gear. OK, I’ve only ever done the Dunwich on SS never fixed but I was on 81″ the first time and 78″ the second time, I think if I’d have gone fixed I’d have been tempted to stick with 81″ just for the downhills if nothing else.
OP, what’s the terrain like? For hilly stuff you’ll have to gear down for the uphills and just put up with spinning like an idiot on the way down but if it’s undulating/flattish, I’d go for a higher gear.