At 34 I trained for London in about 3 months after a friend on a charity got me to fill a vacant spot late on. Nothing too technical about it, just broke it down to a month of regular (every other day or so) 5-6 milers, a month of slightly less regular 10-12 milers and then the rest doing 12-18's once a week. Never ran the full distance til the day but came in just under 3.40.
Not big timing it, just trying to say it's easily achievable - just build up to it steadily in the time you have then eat carbs like they going out of fashion for a couple of days before.
I now run 1/2's for training once a week and one organised one a year - they hurt every time.
I'm sure there's more technical advice coming you way - good luck.