Jay,
I think you are right on. It will take a while. If you read the training section on letsrun, or even happen to come across Lydiard, they both say that consistently applied high mileage takes years to produce great results. I am not saying you won't improve incrementally over time, but when you can consistently handle more mileage and still perform quality workouts without becoming unduly fatigued, that is probably when it will be the most effective.
A real easy way to make sure the mileage takes quicker is to slow down your easy days and add Marathon pace or progrssion runs into your training a couple of times a week. The longer these runs you can handle the better. You should feel like you are running hard but not pushing it. Good luck with the training.
Mike