From the October 2014 Runner’s World, p. 50, we have a common explanation of why most marathon plans only have a 20 mile long run: “That’s far enough to put you in a position to finish the race without elevating the risk of getting injured,” says Joe English. The idea (hope? theory?) is that the combination of fitness, the taper, and race-day adrenaline will carry you to the finish.
On the other hand, Jeff Galloway has a 26-or-bust program, saying “Our surveys show that runners tend to hit the wall on race day at about the distance of their longest run.”
And I would say that Jeff Is right. I made it to 23 on pure adrenaline after training to 20, and I made it to 26.2 after training to 24 (and really, I faltered at 25). And perhaps this is the self-fulfilling prophecy of the 20 mile wall. Train to 20 miles, and guess when the fatigue is going to knock you out? 20 miles. He goes on to say, “Injury risk is close to zero if the [long run] pace is slow enough…” Still, I am aware of the physical limitations of storing about 2,000 calories of ready to burn carbohydrates in your muscles. After that, it’s a long trip to your liver for more carbs, or it’s scavenging something to burn from your muscles (protein) or adipose layer (under skin fat).
A quick google search has left me hopeless: http://home.trainingpeaks.com/blog/article/can-you-become-a-fat-burning-machine
I have thought about the wall and how to train for it. I have thought about running through the wall, transcending the wall, and climbing the wall. Usually it feels like a swamp to me or like running through quicksand. But as I think about taking another shot at the marathon, I’m thinking about pushing that wall. I see the wall as a football sled; and I am the offensive line, pushing it off the line of scrimmage.
I learn through experience and through my body; and for the marathon, I think my body needs to learn by experience what 26 miles feels like. So sign me up for a few more (really long) long runs.