I'd suggest you give yourself a year to train and do at least 4 races of increasing length during that time... 10k, 10 m and 2 half marathons. You'll need to have run for at least 3 hours (18-20m) 3 times before doing a marathon.
Runners world magazine does a London marathon build up training program that has the penguin 'just-to-be able to get round schedule' with a walk-running technique, might be worth a look for a first time marathon....and of course it does give you a look at the training required for faster times if you take to running easily. Can't remember when the training count-down usually starts but I think it is over 3 or 4 issues so could be dec/ jan time? Worked for me. Also gives local races/ other races to target to build up and good tips - as of course do other running magazines!