Hello Val,
I've done two London Marathons. My best time is 4 hours and 17 seconds!!
I've just signed up for my latest challenge. A triathlon
http://www.thelondontriathlon.com/LThome.html
There is loads to think about for the marathon. But the main thing is getting good quality runs. I ran 3 times a week. Two short quick runs and one long steady pace run. And gym and swimming on the rest days.
Have you thought about taking on energy during the marathon e.g. energy gels, banana's, power bars, energy drinks?? Some people can react badly to some energy foods.
Absorb the crowds. I found the got me round!
Well, good luck, I'm sure you'll be fine. Enjoy the day.