You get to your third day of an exercise that your body isn’t used to and you hit the wall: the point at which your body runs out of all it’s immediate sources of energy and has to start the complicated chemical reactions that break down fat into sugars.
You know this has happened because suddenly every muscle in your body starts begging you to curl up into a ball and go to sleep.
For me, this happened half-way up a 700-metre mountain on the island of Harris, on day three. During a hailstorm. And a gale.