The fundraising was always why I never did the London Marathon - did the Southampton one on the same day instead, less kudos but less stress too. Happy to donate a bit to help you along.
How's the training going? When you get past the 15 mile mark in your training it becomes a whole different beast and more about mental strength and the will to push yourself on.
Also:
Don't skimp on the long run. Life happens so you won't always follow your plan, but whatever you do, make sure you do your weekly long run.
Fuel properly. I used gels (use whatever works for you) but it's the same as thirst, don't chase it. I had a gel every half hour with the last couple being caffeine gels - caffeine lowers your perception of effort and after three hours of running you'll take all the assistance you can get from it. Whatever you do, make sure you do the same every time, don't start something new for race day.
Buy more than one pair of shoes. Shoes need time to recover too and if you pound the same pair every time they'll lose their effectiveness.
Recover properly. Recovery's as important as the training, get 20g of protein in within 15-20 mins of finishing your run. Chicken, tuna, bars, shakes, whatever works for you .... maybe not a tuna shake though 🤢
Oh and have fun.