First, as Rita mentioned, the key right now is consistency. Get running, in whatever way you can, on a regular basis right now. This, not the training in the final 2-3 months, is what really determines how well your marathon will go. The more you get in now, the better the results you can expect.
Second, as Rita also alluded to, a lot of people seem to think that the treadmill is great for pace training because you can set a pace and go for it but the exact opposite seems to be true. My thought on this is that people jump on the treadmill, set it to a pace, and don’t really think about the pace they are running. If you are running outdoors, you have to learn how to set your pace yourself because you don’t have a machine setting it for you. That active pace setting you do when running outdoors as opposed to the passive pace following you do on a treadmill is the difference between learning pace and not.
Third, along with what Rita mentioned, there are other benefits to treadmill running. You can train for the heat in the winter (of course you also can outdoors by overdressing) or the cold (if you don’t mind the a/c bill) in the summer. Unfortunately, you will be facing weather conditions in the race and you’ll be preparing in a controlled environment. You can control when in your runs you hit the hills instead of letting your route dictate that. Truly bad weather creates an obvious benefit.