This is about the simplest off road route there is around here. Follow Route 21 out of Redhill towards Caterham, climb up to Gravelly Hill [which takes a bit of effort] then follow the North Downs Way back. 12 or so miles, takes about an hour. Naturally once familiar with the route, there are numerous alternative pathways that can be followed to make the ride longer. You could even follow it all the way in to Docklands. Provides some fantastic views of London... smog permitting that is.