Explain one metaphor or a simile and its effectiveness in the poem "IF""IF" by Rudyard Kipling

Near the end of his immensely popular didactic poem, "If," Rudyard Kipling writes,

If you can fill the unforgiving minute

With sixty seconds' worth of distance won

This metaphor of the minute as representative of the time of a person's life reinforces the previous lines and the theme of Kipling's:  A true man is responsible for his life as, existentially, he creates this life by being accountable for his actions and time.

That the minute is "unforgiving" implies that the metaphoric runner must make use of every second, every moment of his life, in order to have this life("distance" as a metaphor for life) be worthwhile.   This wise and frugal use of time is just one of the conditions that make "you, a Man, my son."