You don't need to run any more than 2 hour long runs to run well at the marathon, regardless of how slow you are.
I have run many marathons, as many as 7 in a year, and gradually found that more than 20 was too much, and my results not as good. From then I reduced to 17 miles in less than 2 hours, just steady runs, and ran them a couple times a week. Three weeks before my fastest marathon, I ran a 2 hour time trial as fast as possible on the flat, and almost threw up afterwards, then I ran the same pace in the marathon over steep hills and where it was 95 degrees at the finish.
The key is to have a balance in your training. For me this meant being consistent with my mileage, 100 miles every week, tick tock tick tock, balanced with the long runs, 1:40 to 2 hours, 2 or 3 times a week, and a couple of marathon speed workouts each week. The mid week one was usually reps up to 4 miles, and the weekend one was usually a long time trial or race. The rest of it was all miles.
I race a 50k in the heat, temperature in the 90s, quite well off running no farther than 17 miles in my training. Mileage and consistence are much more important than the long runs, which are greatly overrated. Your diet is extremely important as well.