Why do we say "Miles to go before I sleep"?

Background:

This phrase originates from the final stanza of Robert Frost's poem "Stopping by Woods on a Snowy Evening."

Frost's beloved poem is oft-quoted and oft-analyzed with interpretations ranging from the literal, that the speaker is stopping to appreciate nature while on a long journey that must be completed before he goes to bed; to the metaphorical, that the poem is about death, or even suicide.

Frost wrote it in 1922 on his farm in Shaftsbury, Vermont. It was published in 1923, the year before he won the first of four Pulitzer Prizes (1924, 1931, 1937 and 1943). Apparently, on at least one occasion he was asked if the poem referenced death, to which he answered in the negative. But, assuming this story is true, it does not negate the interpretations put on it by others, nor the relevance of people from all walks of life quoting from it in the context of death. For example, Sid Davis of Westinghouse Broadcasting reported the arrival of President John F Kennedy's casket at the White House with a passage from the poem, and Justin Trudeau rephrased the final stanza in his 2000 eulogy for his father Pierre Trudeau, the former Canadian prime minister: "The woods are lovely, dark and deep. He has kept his promises and earned his sleep."

You can hear Robert Frost reading Stopping by Woods on a Snowy Evening, and read it for yourself below:

Whose woods these are I think I know.
His house is in the village though; 
He will not see me stopping here 
To watch his woods fill up with snow. 

My little horse must think it queer
  To stop without a farmhouse near 
Between the woods and frozen lake 
The darkest evening of the year. 

He gives his harness bells a shake 
To ask if there is some mistake.
  The only other sound’s the sweep 
Of easy wind and downy flake. 

The woods are lovely, dark and deep,
But I have promises to keep, 
And miles to go before I sleep, 
And miles to go before I sleep.

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