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Not sure the easiest way to get down from where you are but from Leatherhead, ride to Dorking and take the main road west out of town, turn south at Abinger Hammer and go through Peaslake and Ewhurst then to Cranleigh there's minimal climbing over the surrey hills. Depending how close to the start of the Audax you're starting I think Peaslake village stores open at 8 on a saturday if you need to get food/drink.
Which part of London are you travelling from? Looks like you have to go over at least one Surrey hill to get there unless you make a big diversion.