One of the first things I'd want to know is when and where you ran the 2:20:56. If, for example, you ran it at the 2011 Boston marathon, then you should recognize that sub-2:20 on a record-quality course is a significant leap from anything you've done up until now, and your consistent 2:23 to 2:25 marathons along with your consistent mileage for many years suggest pretty strongly that you need a significantly different training stimulus to drop three or four minutes.
My peak years of running came during a very demanding professional schedule, and for me, training was largely a matter of time management and injury prevention while seeking to maximize training volume and a solid level of intensity. One thing that I consider to be a big deterrent to achieving peak marathon performance for someone with a very busy schedule outside of running is regular racing, which is generally a big time sink as well as a constant disruption of efforts to increase training stimulus beyond what you've done before. I gather that you're running five to eight marathons a year, and also racing shorter distances between marathons. That's fine if it fulfills other goals that you have in running and the rest of life, but I don't think it's optimal for someone whose main goal is to make a substantial leap in performance from where he's been stuck for a while.
As for specifics, I'd concentrate on getting peak mileage solidly above 100 miles per week. Yes, there have been many people who have run sub-2:20 on lower mileage, but your marathon history, frequent racing, and relatively slow times at shorter distances suggest that you may not turn out to be one of them. Also, it's a mistake to assume that running less than 100 mpw is an easier route to faster marathon times for someone with regular daily job and family commitments. If you're going to improve on less than 100 mpw, then you're probably going to need to modulate your training more from day to day, because a steady diet of seven- to eight-mile runs isn't likely to get you there, but your work and family commitments may make it difficult to vary your training more during the work week.
I believe that figuring out a way to add fifteen or twenty miles per week is still one of the most surefire ways to improve marathon performance for someone who isn't already bumping up against his body's capacity to handle higher volume. And getting away from frequent racing should allow you to handle higher overall training volume and push beyond previous levels of training stimulus while freeing up time in your schedule.
By the way, I think that treadmill running can be great marathon training, but moderately paced seven-mile treadmill runs generally aren't getting the most benefit from a treadmill. I think that treadmill sessions using steep inclines (for me, probably not less than a 12 percent grade), either as interval workouts or steady runs, can be very beneficial, since they allow a high level of cardiovascular and aerobic stress while minimizing many stresses on bone and connective tissue. I also think that treadmills are great for progression runs of up to about an hour, when you're really working hard to maintain pace and form toward the end.