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There's the Malcolm Firth guide out there that's really good at predicting 12hr distance from 100 mile time. Unfortunate it seems to have been taken down but you might find the distance table if you search through TTF. Was pretty spot on for me choosing to target 270 miles.
Said guide promotes the whole slowing down schedule which is nonsense. As even as possible is best. The finishing circuit that you spend 2/3 hours on is slightly faster than the main loop which will help with fatigue.
edit - doing the work for you
Remove stock pads and get some super thick foam I guess? 3T are pretty good pads though.
Mostly joking though, I've never actually had forearm issues on the A31, but some people come away with bruises.