Very well done on your marathon. Im well impressed! Hopefully one day ill be doing one.
I dont think your age is a stumbling block at all for you to hit a high standard. The worlds your oyster is what i say! Gd Luck.
As for clubs, i noticed earlier on cause they were being interviewed on the olympics. There is one called the Serpentine athletics club or something. I think they train in Hyde Park. Supposed to be very gd.