Why 18 Miles is Hard

This is an article from About.com

The marathon is a challenging running event because of its duration. After two hours of running, by the 20 mile mark for fast runners, the body runs out of glycogen (stored sugar) and begins breaking down the protein in muscles and tissues for fuel. They "hit the wall." The body should convert over to burning stored fat, but can't because some carbohydrate is needed to allow the burning of fat, but it is all gone.
Walkers, other than racewalkers, experience this less because at a slower pace the body uses fat stores for energy throughout the event rather than turning to burning up its own muscles. Walkers are likely to get progressively tired throughout the long distance, but without the paralyzing experience of hitting the wall.

A walker can take from 4 hours to 9 hours to complete a marathon at a walking pace. That is a long time to be out in the elements, exerting yourself. Blisters, chafing, sunburn and heat illness are common hazards. Those who aren't careful to drink enough water and electrolyte replacement drinks during the race will feel the effects of dehydration.

After a marathon, the strain on the body is apparent. Even without walking at a speed that leads to burning up muscle, minute tears happen in the muscles and there is build-up of the toxic breakdown products from exercise - lactic acid, etc. Muscle strains and sprains are more likely due to fatigue during the race.

But by being so tough, marathons also become addictive. While some people can do "just one," and most vow never to do it again after their first, plenty of people get hooked on seeing if they can improve their times from year to year.

Hal Higdon, who has run over 100 marathons, writes, "In a marathon, you don't beat others. Instead, you achieve a personal victory." It is a very personal event, each participant having their own goal to achieve, often just to finish.