The Boston Marathon will be run later this morning and my good friend John Ruberry, also known as Marathon Pundit, has run this race three times among his dozens of marathons. He has a good description of hitting the wall in Boston.
At the beginning of the race in Hopkinton, the elevation is 500 feet above sea level. The first 16 miles of the race, until the town of Newton, runners enjoy mile after mile–with an interruption now and then–of downhill racing. At the Newton western town limits, the elevation is just 50 feet above sea level. But then, thousands of legs, accustomed to miles of down hills, have to contend with four miles of uphill pacing–finishing off with the storied Heartbreak Hill at mile 20.
Now about mile 20: Even in fast and flat marathons such as Chicago's, the vicinity of the 20th mile is where many runners "hit the wall." They get a somewhat sudden feeling of severe fatigue. Legs, up until this time which are fairly limber, become stiff, and as many runners phrase it, become as flexible as telephone poles.
At Boston, throw downhills and sudden uphills into the physical fray, and those same legs, rather than being telephones, feel like steel corkscrews.
I have completed two marathons and had to stop in a third because of a bad hamstring. All on flat earth and at elapsed times that did not qualify me to torture myself on the hills of Newton.