Not sure if there are any around HOP but there are two based in Dulwich. The Dulwich Runners and the Dulwich Park Runners who will likely have others training for marathons.

I've just finished training for a half marathon but just using a training plan from runnersworld. I'd guess though that discipline wise to get up to the 40 or so miles a week, you'll need to do, a running club might help! Good luck