This article in Running Times gives some advice to help improve your endurance in the marathon. If I could give advice to anyone training for their first marathon or 100th marathon, it is to log more miles. If your running 50 miles a week and not improving, try running 60-70. It even makes sense from a physiologic standpoint. You need your body to adapt to the distance it’s going to be running. You wouldn’t train for a 5k by only running 3k, right?  I’m not saying you need to reach distances of single runs of 26 miles, but if you are going to be running a race that is going to last up to four hours, your body needs to get used to being on its feet that long. 

When you work with marathoners as fast as 2:10 and as slow as 6 hours, you learn a thing or two about what makes for a successful race. The training might be different depending on the goal, but every runner needs to consider three crucial factors. 1. NUTRITION

Source: Nutrition, Fatigue & Fast Finishes