Marathon Questions: How Long for longest run

I am doing a short series of posts on questions I frequently hear at run club and seminars about training. Let me preface this by saying that I am training for my first marathon; I don't have a vast font of experience to draw from. I am not an expert, but I am someone with a good memory, research skills, and I read a lot. (see here for a discussion of whether anyone should listen to me. . .)

The most common question is: how long should my longest training run be for a marathon.

It is the great fear of first-timers:

"I was dying at the end of 20 miles; how will I make it to 26.2?"

"Shouldn't I run longer?"

The answer is 20 miles. Now, I spent most of last fall thinking this could not be right. But it is!

I started by reading looking at EVERY book in my library and the local bookstore on running (Barnes & Noble, Fargo ND). I was AMAZED to find that no training plan for beginners had a run longer than 20 miles. Higdon, Galloway, Runner's World, even Beardsley had 20 miles! Why???

Well, the reason is pretty easy to understand, once you think it through. The human body has enough glycogen for 20 miles. When you run out of glycogen, you "hit the wall." The fact is, I can run the day after a half-marathon; but every expert says plan on 4 to 12 days of no running after a marathon. Imagine having that in the middle of your training! Your chance of injury goes up significantly when you are out of glycogen. Even runners that finish a marathon "injury free", have micro tears of their muscles.

How about advanced runners? There is MANY different opinions here, but there are common notes. Most advanced plans I reviewed stop at 20 miles (Dick Beardsley's from the Grandma's Marathon site has one 24 mile run). The rest increase the number of 20 milers (Higdon has one for beginners, two for intermediate, and three for advanced). Also, most programs build by running parts of the long runs at tempo pace, increasing weekly mileage, etc.

That is the second part of the answer. More weekly mileage. If I was to summarize the books I have read, it would be:

1) Longest run 20 miles. Anything more and the risk of injury outweighs the benefits.

2) The final 6.2 miles are trained for by your weekly miles and base endurance.