Every year hundreds of thousands of runners will have the opportunity to “hit the wall.” “The wall” is defined as that period in a marathon or long distance event, when things transition from being pretty hard to being really, really hard.
Hitting the wall is basically running out of energy. For the first couple of hours/miles of running you are using up all your carbohydrate/sugar stores. After these are used up, your body starts to use fat as its energy source. ...