The end of October is pretty close in terms of preparing for a marathon. There is obviously a huge difference in preparing to come in first place and preparing to be able to finish the race without looking like a fish that got pulled on the shore.
For the first month or so I would recommend a cross training exercise, such as Soccer, Lax, anything that forces you to be constantly on the go. You'll find that you can improve your overall fitness level and cardiovascular capacity while having a bit of fun at the same time. If that is not an option then lace up your shoes and start pounding the pavement. Start small and work your way up, but remember you don't have a lot of time so striking a balance between not killing yourself in preparation and being able to finish the race is going to have to be set by you.
Running can quickly take a huge toll on the body, so swimming is also a recommended activity on days when you can't run or need a change of pace. Running 3-4 days a week and swimming 2 would be a good mix, and take some stress off your joints.
Train on the same surface you will run on during the race as much as possible, although running on beach (not the wet sand) once a week will give you a pretty good boost as well. Also, try to run on even surfaces as much as possible. If this is not possible try to alternate the direction you run in daily, as this will help reduce the appearance of shin splints and ankle related issues.
If you decide to incorporate weight training into your routine, avoid doing a heavy cardio workout on the same day as this will reduce the weight trainings benefits.
One final tactic is called carb loading (or carboverload, etc). Eat good, natural sources of complex carbs daily (wheat, grains, etc...nothing refined or packed with sugar). A week before the marathon, cut them off. Two days before the marathon, begin eating them again. Your body will hoard the carbs because it hasn't had them lately, and when you go out to run your race you'll have an extra stockpile of gas in the tank.
Hope this helps and best of luck to you.