I do a fair bit of running Martin, not run a marathon.....to far for me and tbh i wouldn't bother unless i could beat my old mans time of 2h36min, he would never let me forget it otherwise. I run half mara distance quite regular, every sunday really.
You'll probably need.... to RUN a marathon -
1 long run per week of 15-20 miles and a couple of mid distance of 7-8 miles(ish). Try not to run on cosecutive days and consentrate on time on your feet rather than speed.
In between your running do some cross training, something low impact would be best....cycling, swimming, rowing.
I'd definatly try some 10k's, 10 milers or halfs before you do a full.