what is the total length of the trial mentioned in paragraph two? why do you think the author wants to be specific?

Here is the quote where distance is mentioned,

"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."

This is so the reader understands the vastness of the landscape and the conditions the man is foolishly travelling in.