What is the total length of the trail metioned in paragraph two? Why do you think the author wants to be so specific?
The little blurb with which the grim story begins, is all the foreshadowing you need:
"He was quick and alert in the things of life, but only in the things, and not in the significances."
Yes, the man was smart about the particulars, the simple facts, but not about what they meant in terms of life and death. The passage you referred to contained these particulars:
This dark hairline was the trail---the main trail--that led south five hundred miles to the Chilcoot Pass, Dyea, and salt water; and that led north seventy miles to Dawson, and still on to the north a thousand miles to Nulato, and finally to St. Michael, on Bearing Sea, a thousand miles and half a thousand more.
The description of the trail length merely hints at the vastness of the frigid landscape that is the setting of this tale. To this expanse of "pure white, rolling in gentle undulations" add two facts: a sun that will not rise and barely lights the sky plus a temperature of fifty degrees below zero. Together they add up to a danger that the man cannot fathom.
All those miles and that "tremendous cold" were known as facts to the man... distances and degrees ... what they implied about his journey, he could not, to his peril, comprehend.