It seems to be a tough objective for a first try.
What are your PRs at 5k/10k/half?
Yee, it will be extremely tough...
Please note: no PR's at 5k/10k/half... never run in race. Ok, may be before marathon I will try some races like 3k-21k as well
going from 9.25 3k after 4months of serious running to 2h20 marathon 6-7months after seems impossible.
I did the same as you : 9.32 with poor pacing 3k in august after 4months base and there is no way I would be able to run 2h20 in february whatever the training.
2h30 would already be a very very very good achievement.