I don't think 12 miles a week is enough.
I would rather see you run slowly 5 miles for 4 days a week (20 miles a week.) When you can sustain that for 3 weeks begin adding 1 mile a week to one of those days until you reach 14 miles. So you would have 3-5 mile day + a long run, all run slowly. At that point run a half marathon. You probably could do this in 3-4 months.
At that point you would be at close to 30 miles a week, would have one 14 mile run, and would have more aerobic canditioning. Aerobic conditioning is cumulative. You want as much as you can get, without getting injured. At that point I would look for a 16 week program for the marathon, and would consider adding a day, making it 5 days a week. I don't think you'll be able to jump from 12 miles a week to a marathon in 13 weeks. I think the long runs would discourage you without the aerobic conditioning beforehand.
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