GOOD LUCK!
Haven just done the marathon on sunday, i say go for it! I have watched nearly each and every one, whether on tv or outside my flat (when i lived in the docklands) and always aspired to do it. It was great. (if you ignore the pain!). I would love to do it again, and be slightly less delirious so that I could soak up the amazing atmosphere all the way around. There is nothing like doing a race where you have so much support for the public.
I've entered the ballot twice, and not got a place (got a fleece that was so cheap and made of hideous material I couldn't even touch it - yuck!) I got my place through a charity and needed to raise £1700.
I'd say start running outside now, whether or not you get in. If you leave it until October, when you will find out if you got a place or not, you will find the marathon harder.