What is meant by "the unforgiving minute" in the poem "If" by Rudyard Kipling?

The stanza goes as follows:

If you can fill the unforgiving minute 

    With sixty seconds’ worth of distance run,

Yours is the Earth and everything that’s in it,   

    And—which is more—you’ll be a Man, my son!

As you know, these are the last four lines of the poem. The "unforgiving minute" is metonymy, or the substitution of a name for an attribute, when referring to time.

Minutes are "unforgiving" because, if we waste them, we can never get them back. A minute that is lost is lost forever. Therefore, the paternal narrator encourages his son to make good use of that minute. Because the poem triumphs what would have been masculine virtues in the nineteenth century, he encourages his son to use that minute on a physical pursuit: "sixty seconds' worth of distance run." 

If his son can manage his time wisely, in addition to following the other advice in the poem, Kipling believes his son will have all that he desires in life ("yours is the Earth and everything that's in it").