Do your weekly long runs.Your weekly long run is the best training to avoid hitting the wall. By doing progressively longer runs each week, your body's capacity to store more glycogen within the muscles increases. By increasing your glycogen stores, you'll be able to maintain your pace and hopefully push off the onset of fatigue. In addition, the long runs teach the body to tap into and utilize energy reserves from fat storage sites after the glycogen stores have been depleted.
Run at least one 20-miler.To make sure you're really getting the long run benefits described above, try to complete 20 miles as your longest training run. It's not necessary to run more than 20 miles because the potential negative effects of running longer than that in training really out-weigh any possible benefits. Follow your marathon training schedule and make sure you're not undertrained for the marathon.
Train at marathon goal pace.
If you're shooting for a specific marathon time, you should focus on your goal race pace during training. You definitely don't want to run your entire long runs at marathon pace (MP), but it helps to run the last 1/3 to 1/2 of your long run at your anticipated marathon pace. Running at MP towards the end of your run is good training because you'll be picking up the pace when your legs are already fatigued. Your body will become more familiar with and more efficient at running at your goal marathon pace.
Don't go out too fast.One of the biggest rookie mistakes in racing is going out too fast in the beginning of the race. Most runners have at least one story about a race when they felt so great during the first few miles that they ran ahead of pace, only to crash and burn (hit the "wall") during the final miles. The problem is that if you go out too fast, you'll burn through your stored energy too quickly and your muscles will fatigue faster, leaving you feeling tired and depleted toward the end of your race. Follow these tips to avoid going out too fast.
Consume calories during your marathon.When you run for under 90 minutes, most of your energy comes from stored muscle glycogen. But if you're running for longer than 90 minutes, the sugar in your blood and liver glycogen become more important because your stored muscle glycogen gets depleted. Fueling with carbs during your marathon will prevent you from running out of energy and hitting the wall, while also boosting your performance. Get tips on what and when to eat during your marathon.
Also see: Marathon Running and Training Mistakes to Avoid