By the time he finally felt the land under my feet, he was tired out.

1) by the time 等到；到……的时候 The kind man man had gone by the time I woke up. 当我醒来时，那个好心人已经走了。 2) be tired out 筋疲力尽 He was tired out after his long trip to Yangzhou. 至扬州长途旅行归来后，他觉得非常疲倦。