Just take it slow and easy. Don't increase your total mileage by more than 10% each week. Learn that you can slow down your long runs to increase your mileage but you can add in intervals during the week if you do have a time goal for the marathon. You seem to have a good fitness base, so increasing your long run each week should come with a lot of hitches (I say that hesitantly because things happen).

I have not yet made it to running a marathon. Apparently being stubborn leads to more injury, which is why I'm here trying to take up cycling!