I am no expert.....running London as my first and my training is not exactly a recommended way of doing it (from nothing to 7 -- 13 -- 12 --- 20)
I suspect it depends on your base fitness and what you want to achieve from it
They say that if you can run 16, you can get round
If you are out to run it, with a time target, then you probably need to forget it. If you want to go out there to get round and enjoy the occassion, then you could think about run/walking it, taking your time and enjoying the party. Certainly I wouldnt leap straight in from being injured to doing a 20miler to catch up with a training plan. If you are on a ballot place, you can always defer and train for next year. If you are on a charity place, I am not sure if you can defer
Sure someone better qualified and experienced will be along to answer soon