Trixxie,
It will depend on the nature of the muscle damage but it may take a couple of weeks of treatment from your physio. If you do more running it doesn't mean you can't complete a marathon but ambitions for times will go out of the window.
Realistically you need to assess what you want from the event - if it is to satisfy your charity obligations you will get around if you take it very steady and use one of the run/walk techniques - the RW pacers do this for the get you around groups and they still do 5 1/2 to 6 hrs.
Once you have satisfied this need and then want to fulfil any ambitions to run a marathon well then choose another lower key marathon to do in the autumn so there is less mental pressure than surrounds doing London and giving you time to get over the injury niggles and train through the warmth of the summer which be will easier on the muscles than running in this constant cold wind.
On race day I'll be working for the BBC commentating on the Interactive Channel on the mens elite race. Once that finishes at 12-noon I will then shoot down from Sheperds Bush to the Embankment to take in the atmosphere and then have been invited by Dreams Come True to visit there post-race function as it is their 25th Anniversary year and we are organising the Dream Miles for them in Windsor Great Park in June......so pretty busy
I'll also be on our stand (2:09 Events) for the duration of the Expo before the marathon if anyone wants to pop along for last minute support or just to say hello.