The main aspect relative to training for one's first marathon is twofold, IMO:
-Developing an aerobic base
-Getting your body used to working for an usually duration of time
The Galloway method is a very slow and steady way to build up miles/time running, you may want to look into that. The way it is programmed, CFE would best benefit a distance runner/triathlete that already has a sufficient aerobic base. Gordo Byrn wrote a book called Going Long; although it is intended for ultra distance triathlon training, there are several outstanding nuggets of info regarding the run portion of IM training.
I'm not sure how much strength you would actually gain while on this hybrid + marathon program. Assuming you would have one LSD running day (long slow distance training), I would figure that would put your body in a fairly catabolic state. even with 2-3 solid 531/short MCs days during the week; a constant stalemate within your body might be the best you can do....
Sidenote: I have several half IMs and one IM finish, all at a bodyweight around 225#. I'm now just finishing up my third 5/3/1 cycle, and I'm at 240ish. I usually do one half-mary every Feb, and finish right around 2hrs give or take. Next Feb, I am looking to run my first stand-alone marathon, and I would like to keep doing strength training, not unlike the OP. Realistically, I may have to put the strength gains on the back burner, while I rev up my aerobic base. Ideally, I would like to keep my strength losses at a minimum. I'll start this programming around Thanksgiving methinks...