My only guess would be to look at local races and see if you can make out any routes from that. There is a Dorking 10 which is a decent race and an 8 miler (Ballbuster?) over that part of the Downs. There is another North Downs run based at Gravesend which is around 19m (30k) and worth doing.
Websites for these runs might have the course outlines.
Comments
Websites for these runs might have the course outlines.
The Ballbuster (if it is the same route as the Ballbuster dualthon) is on road. I know the roads a little round there.
The Surrey slog also went just to the South of Box Hill