Hi ami, 2 things really, and as a new comer to marathon running, any one in here can tell me I a wrong, but can only speak from experience.
I am 39, started running seriously 18 months ago, have done lots of 10 k races, quite a few halfs, PB 1.47, and have just done Chester as my first marathon. I felt it was the perfect race for a first marathon, not to big, not to small. I had been thinking about doing London, but probably won't. After all the months of training, I think I would be frustrated and disappointed if I was in a race with 35,000 other runners. If are serious about running, as opposed to just doing one marathon, I would suggest doing a race that maybe only has up to 10,000 runners.
The second thing is, I personally don't think there is much of a substitute for miles in your legs! I will run more during training for my next marathon. Eat well, booze less and sleep more. Simple!