He was too good a borderer now to exhaust himself in the beginning.
He was overpowered after a while by an immense lethargy.

A great collapse, both physical and mental, came after so much exhaustion.

He felt that he must rest or die.

The night was mild, as the spring was now well advanced in Texas, and he sought a dense thicket in which he might lie for a while.

But there was no scrub or chaparral within easy reach, and his feeling of lassitude became so great that he stopped when he came to a huge oak and lay down under the branches, which spread far and low.
He judged that he was about six miles from San Antonio, a reasonably safe distance for the night, and, relaxing completely, he fell asleep.
Then nature began her great work.