Congratulations. Am also doing Ride London and will be taking in the three climbs next weekend - Newlands, Leith Hill, Box Hill, time permitting. Done Leith Hill and Box Hill together and done Shere and Leith Hill but not three climbs one after the other, so that'll be fun. Will also do the wee climb up to the Muddy Duck as a warm up
Speed wise, I made a decision to try and stick above 15mph as much as possible to raise my average and it seems to be working. Did a 33 miler (including the Weybridge to West Horsley segment of RL) yesterday at average 16mph which was a pleasant surprise. If I can manage that on the day then, hopefully, there should be enough redundancy embedded to cater for punctures, hills, and stops...did I mention hills?