I've done London 4 times....run it twice, walked it once and run/walk once. Fastest time was 4:44, slowest was just over 7hrs!
Its not the actual course that kills you, its the training! Put the training in, and you'll enjoy the day (well, sort of!).
I used to dread the long runs in training...its ok for the whippets who can do 20 miles in 5 minutes, but for me it was more like 4 hours, in all conditions!
Now I do no excercise at all and cant think how I used to do it!
So best wishes to you and I hope it goes well and you have a great day - the crowds are amazing.
Which charity...hows the fundraising going (I always found that harder than the training!).