There are really two ways in which runners "hit the wall". One is when lactic acid builds up in muscles causing not only pain but decreased performance. This is due to muscles using anaerobic metabolism when there is not enough oxygen being brought to them. This is typically what you'll feel at the end of an 800m race.

The second type of "wall" that can be hit is when the body runs out of glycogen and has to start burning other things for fuel. This is what you will most likely be feeling at the end of a marathon.

In no case do I believe that "Runners are often at their fastest at the end of a race, when they should be the most tired." In fact I'll call bullshit on that one.