The Question and Answer section for Call of the Wild is a great resource to ask questions, find answers, and discuss the novel.
From the text:
Since the beginning of the winter they had traveled eighteen hundred miles, dragging sleds the whole weary distance; and eighteen hundred miles will tell upon life of the toughest.
They discovered yellow metal or gold!
John Thorton suffered major damage to his feet from frostbite. He needed to take to take it easy and heal.... as did Buck. For awhile, they cease their wandering to nurse their wounds.