The Surrey Hills loop up there ^^ doesn’t have much tarmac at all apart from the bit between the bottom of Newlands and the canal in St Johns, which is about 5 miles.
There is a way home from Newlands which is entirely off road, but it is 10 miles further and a lot slower. It basically heads out to Aldershot on the NDW via the Hogs Back and cuts home via Ash Ranges, Tunnel Hill and Porridge.
There are also ways of linking up Leith, Box and Ranmore but I tend not to ride over that way much as by the time I get to Leith and start heading home that’s far enough East for me… 100 miles is quite enough. I don’t do driving to ride.
Yes, 8hrs is pretty quick for that distance, those trails and that much climbing.
GB