I had a 14:50 5k PR in college, and 1.5 years out, debuted with a 2:22 marathon. Trying again in the spring. Granted, I ran 100+ mpw for three years before the attempt, and got up to 120... So, like many are saying here, it's not impossible, but you will need to keep bumping mileage.
Keys I've found:
Steady long runs @ MP +20-30s/mile
Practice fluids on marathon-specific tempos
Spend a lot of time running MP
Trading for the 10k/half during base period
Double at least four times a week
Flexibility makes you inefficient
No need for two hard sessions and a long run every week. Many times, I do one hard session and one steady long run a week.
Do more intervals (i.e. 10xmile instead of 4-6)
Get ready for major quad destruction and loss of cognitive abilities (special blocks help)
I would suggest focusing on getting your half marathon and 10k times down over the next 12-18 months, then debut in the full. Or, if you're going for the full soon, go out slower than you think you can run and try to make the last 10k the fastest portion of the race.