The long run is the meat and potatos (pardon the analogy coming from a vegetarian) for the marathon. Drenth, in her Marathoning for Dummies quips:"Miles are to the marathoner as beers are to the bowler." For the beginning marathoner, the long run is what builds your strength. The long run builds your endurance and inures you to the dreaded Wall. It changes you physiologically and biochemically: this is where I get my satisfaction as a marathoner. The toughness and strength that repeatedly breaking 20 miles gives you is a wonderful feeling for a 51 year old man who used to be nearly 30% body fat.

Various authors report that long runs, specially when training for the marathon, affect the way your body metablolizes energy. I've read that these runs increase mitochonrial count (these are the organelles which create the energy muscles need to contract), cause fat metabolism to occur earlier in the run, and increase the efficiency of fat metablolism. These last two effects are important. The body can only store 1,500 to 2,000 calories in the form of glycogen and glucose. This is not enough to run 20 miles.

The body is always using some mix of fat and glycogen/glucose for energy when running. Glycogen/glucose are the primary short term stores in the body. The body is always using some mix of fat and glycogen/glucose. When running, the ratio of glycogen/glucose to fat is proportional to running speed. Run faster, and you will use up the glycogen/glucose faster because the ratio of its use is higher. Likewise, if you run more slowly you will use more fat. Fat metabolism is much less efficient than glycogen metabolism.

"Hitting the Wall" is the sudden switch from glycogen/glucose to fat metabolism because you have depleted all of this source of energy. This can happen when you run run further than you have trained and your body has simply used up all its glycogen/glucose. It can also happen if you have run too fast relative your training and your body has not been able to manage the the loss of glycogen/glucose relative fat in a graceful way. The more you run long, the more your body adapts to an earlier, more graceful use of fat as an energy source. Thus, right combination of pre-race training and pace judgement during the race will enable you to to "hit the wall" right at 26.2 miles or avoid hitting it at all.

For my second Marine Corps Marathon in 2006, I ran over half a dozen long runs in the 20 to 23 mile range. Before this, around April, I was hitting the Wall in the middle teens on my runs. By the end of the summer, I could do 22 miles with impunity. This ability had a substantial impact in the last 6.2 miles of my marathon. In 2005, I hit the Wall at mile 20 and was reduced to the dreaded "marathon shuffle". (This is still running, but half stumbling forward, too.) In 2006, while I got very, very tired, I was able to maintain my open, running stride. But let there be no doubt, at 26.2, there's nothing left!

Running long makes runners stronger in shorter distances as well as the marathon. Here are some recommendations from Pete Pfitzinger :

(from Running Times Magazine, Essential Ingredients III: Long Runs, Pete Pfitzinger, 05/2006.)

Pfitzinger goes on to state: "Novice marathoners with the goal to finish should include no more than one to two 20-milers in their marathon training. To be optimally prepared, more experienced marathoners should put in about five to eight 20-milers, including two to three runs of 22 miles. The last 20-miler should generally be three weeks before the marathon."