The old saying "Everything in Moderation" is a classic, but in my opinion it leads only to being average. Being your best is a choice, and consistently making good nutrition choices leads to peak performance.
We all have a choice of how we want to eat. We know what is best for us: simple, natural, nutrient-dence foods. The challenge is choosing to eat that way, making it a habit, and sticking with that choice. It takes discipline, focus, and awareness.
Not to sound like a drill sergeant, but too many people say they want to be fit, healthy, thin, toned, and low in body fat, but they do not have the awareness or are not willing to do what is necessary to make it happen. Your actions need to match your goal. In training, you cannot expect to run a sub three-hour marathon by training twenty miles a week. The same equation works with nutritional health; consistency reaps the highest benefits.
Keep your pace steady in eating well, and I guarantee you the benefits will be profound. It's not discipline for its own sake. Practicing discipline makes us feel good about ourselves, and this empowers us further. Ironically, it is the pleasures and comforts we're scared to lose that ultimately lead to unhappiness.
You have a choice as to how you want to eat, how far you want to dedicate yourself to improving your diet, same as you do with your training. The better you eat, the better you will feel and perform as a runner and in life. Nutrition has a cumulative effect, just like good form and fitness.