Thames hare and hounds springs to mind first, based on Wimbledon Common/Richmond park, not so far from the Oval. Wimbledon Windmillers seem to have something going on most nights. Clapham Chasers are the closest club as far as I know to the Oval but not so off road as the two previous clubs. For central London try Serpentine, but again, mainly road. For SE London look to Blackheath Harriers, Dulwich runners or Kent AC. Dont know much about them though. There are dozens of clubs in SW London and I'm sure most will welcome you to join 'em for a run where I'm sure you'll find like minded souls. Have a Goggle at 'running clubs in South London'. Good luck.