“When the morning’s freshness has been replaced by the weariness of midday, when the leg muscles quiver under the strain, the climb seems endless, and, suddenly, nothing will go quite as you wish – it is then that you must not hesitate.”-- Dag Hammarskjöld
I found this in a discussion about discipline. In fact it works as a good definition of discipline. Note that Hammarskjöld is not saying that you cannot rest if you are absolutely exhausted. What he says is that you must not hesitate when you are dragging.
I interpret this as meaning that you must not, during a time of weariness, take that as a time to question your path or make major decisions about whether to proceed at all. It is exactly at the times of weariness that we most need the discipline to carry on as best as we can, trusting the path we had previously set for ourself, and trusting in God.