I also ran London in 2012 both times I got a ballot place but have never managed since. There is no way to run London except for a ballot place or a charity place and both are quite hard to come by.
Once you get a place VLM are very good at sending you information. Lots of blogs about training and nutrition. The information on how to get to the start and the public transport is great. There are 3 start sections so it’s really important the you get to the right one.
The run it’s self is very well supported. There is the Tower bridge at the half way point and if you are a 4-4:30 runner you will see the finishers heading up the other side of the road to 22 mile point as soon as you e crossed. This is where I saw Mo Farrah. I was half way he was off to the finish