Wow, well done, I'm envious of you.
I've only done a 5k so far and am working my way up very slowly due to physio's orders to a 10k hopefully next spring. It's my dream to run the London Marathon one day but it seems so popular that I don't know if I'll ever get a place.
Yes, just ran my second marathon on Sept 18. London sounds like a great race!! You have 6 months to train, a good amount of time. Congratulations on getting in!