I honestly don't know whether to be appalled at or impressed by your diet!
I think that diet is an important part of overall running fitness. I would rank it as a one of the key elements, after training volume, efficient training intensity and sleep. I would put it above stretching and strength training absent a tendency toward injury.
That being said, I ran all of my PRs in college essentially eating a diet like you describe above. But my subsequent resurgence as a masters runner has been, at least in part, fueled (pardon the pun) by a good diet. I still eat bad foods (pizza every Friday night with wife and kids, wings and fries every Saturday afternoon for lunch, occasionally throw down a big bag of potato chips, drink too much beer, wine and whiskey for sure, eat a couple of hundred calories of chocolate every day), but the core of my diet is balanced and nutritious. It started off as part of a need to lose weight in order to have a viable comeback, but over time it transitioned into something that I have to do to feel good most days. I just feel like crap when I eat crap. And I think our margin for error as we mature gets narrower.
I figure that about 95% of running success comes from running miles, getting in good and efficient workouts, getting enough sleep and eating well. The other 5% comes from the other things I mentioned - strength training, stretching, drills, massages, etc. Of that 95%, I cannot say what percentage I attribute to a good diet, but the more overweight you are, the higher that percentage is. You are a fine weight for your height generally speaking, but for a marathoner you are probably on the high side of your acceptable range. A good diet that helped you get to a leaner body composition would probably go a decent way.
My typical day (probably 4-5 days a week of this):
Breakfast - yogurt and a hunk of cheese.
Lunch - Sandwich (usually chicken, ham or turkey) with no dressing, soup or salad, iced tea.
Snacks - Fresh fruit (usually berries or grapes), pretzels, almonds, raisins, chocolate.
Dinner - Lean meat (chicken or pork), whole grain, steamed fresh vegetables.
Of course, I am lawyer and not a nutritionist or dietitian, so what do I know?