Enjoy! I did it in January of this year although for the first time ever I pulled out - at 19 miles. I was so obsessed with getting up at 3.30am for the 6am start that I went to bed at 8pm and then just couldn't sleep. I never slept at all and felt dreadful at the start. At 6 or 7 miles I thought maybe it was going to be OK but felt dreadful at 13 (how I envied those peeling off to complete their half marathons)and I finally pulled out at 19. All of that said, apart from the early start it is a fantastic event. The music, characters, crowds, parks were memorable. I can't help you with fund raisung as I've got £1,250 to raise for the FLM but thought you might be interested to hear of my experience of it.
I'd love to do it again one day - properly!
Good luck.