I ran London last year - October. Was hoping for sub 4 hours and came in just over 4.
I found it difficult to get going the first few miles. There are so many runners that you find yourself dodging in and out. The crowds are great throughout the 26 miles. However the last 3 miles or so, there are that many people in the crowds that you are running through a wall of noise. This did wipe me out a bit as I just wanted to keep my head together and get to the end, at that point.
It's also worth mentioning - At the finish you are steered through to St James's Park. I'd suggest to avoid dizziness and so forth, make sure you have plenty of refuelling in your drop bag as it took me over 2 hours to exit the park and get to the underground station. The crowds are huge and you're effectively kettled through to the exits in stages.
It is a great day - Hope you have a brilliant race. As @Rob kmentioned - Build slow and you'll be fine!!