How quickly can you train for a marathon?

How quickly can you prepare for a marathon - I don't mean an all out attempt at smashing PB marathon, but a get you round in one piece attempt.

Reason for the question is that I have been pretty much out of running action for almost 4 months due to Plantar Fasciitis and today I have had a cortisone injection in my heel, I have my leg in plaster for a week and I'm hoping that this resolves my problems.

I am entered in the New York Marathon in November and it is all paid for - flights/accomodation/race entry etc and as you can imagine this does not run cheap.

I have 99% resigned myself to having a hols and watching the race, but is there anyone out there that thinks there is still time to get ready to get round.

I have been cycling and swimming during the non-running period and my general fitness is ok.

 New York Marathon is7th November.

I ran this years VLM in 3:18 but would not even dream of anything within an hour of that time - in fact walk/run is certainly an option.

any comments welcome.

Depends on what you meen by get round, think it's possible to do the race but you might have to walk some of it and if the injury hasn't been fully sorted you might risk long team injury. Under normal circumstances would say 3 months but sounds like your fit and fast which may have a big inpact to make it less. But also sounds like you've had bad luck which might make it the other way around?

When do you have to decide? Can you do some light training and judge is later on this year once you've sorting out the plaster? 

I can of course defer - but that would mean forking out about £800 for another holiday in New York next year and paying the entry fee again (which is over £200) - which I really don't want to do.

Cancelling the holiday this year is not really an option as stupidly I didn't take out holiday insurance at the time so I would lose all my money anyway.

I can delay the decision to run/not to run right up until race day I suppose.

 More than happy to walk / run - it is only about actually doing this event and seeing the city from places the usual tourist wouldn't - the race goes through Harlem, The Bronx, Queens etc all places I guess I wouldn't go whilst not running the marathon.

I have sort of resigned myself to not being able to do it - I was just hoping to hear of some miracle method.