In Rudyard Kipling's poem, "if", what do "unforgiving minute" and "worth of distance run" mean?

The full-length poem is here.

I love this poem and know it by heart, but I don't fully understand the following verse:

If you can fill the unforgiving minute
With sixty seconds’ worth of distance run,

I've looked up unforgiving in the dictionary, but I still don't understand how the word relates to "minute."

And what does "worth of distance run" mean?

My take is as follows:

A person has one life to live. In the poem, "the unforgiving minute" is a metaphor for the amount of time people have to live. That minute, the total time people have to live, is unforgiving because time doesn't give anyone a second chance. Once a second (60 seconds in a minute) passes, it is gone forever.

If you can fill the unforgiving minute (fill up the precious time one has to live)

With sixty seconds’ worth of distance run. (with a life-time's worth of hard effort)

In the second line, the author is telling readers to "fill" their lives with efforts that they would be proud of, in the way a runner would "fill" their sixty seconds (minute) of running time in a race with as much distance run (the amount of distance run) as possible.

The other answers correctly highlight, I think, the fact that time is unforgiving in that time passed is gone forever. But why the 'minute' instead of some other measure of time? Here are two interpretations that seem plausible:

(1) The minute is unforgiving in a way that the hour or the day is not because of how fast it passes – a moment of distraction and a minute is gone. Hence the exhortation to occupy every second of every minute with whatever task is at hand ('distance run'), because even if 60 seconds doesn't seem like much time when trying to complete most tasks (including running), you'd take much longer to accomplish anything if you're not focused on the task for all 60 seconds in each minute.

(2) The minute might be unforgiving in another way if the task at hand is arduous or painful – 60 seconds is a long time to endure if you're having a terrible time, but at the same time a minute is such a short period of time in the context of most arduous or painful tasks (imagine running a marathon or doing a repetitive menial task). But the only way to get closer to your goal is to fill all 60 seconds of every minute with your fullest effort.

Of course, in both cases this image resonates more clearly with some things in life than others.

"Sixty second's worth of distance run" is a reference to the Christian (and to a certain extent particularly Protestant) image of a human life being like long-distance race requiring commitment and great effort. This is exemplified by the first line of the second verse of the hymn Fight the good fight which is

Run the straight race through God’s good grace,

The image comes mainly from the writings of St Paul many of which are presented here and it fitted the Victorian view of life with which Kipling and his target audience would have been raised.

What Kipling is saying is that, if his readers can concentrate, keep the faith and commit themselves so completely to living whatever Kipling thought of as a "good life" that they make as much progress as they possibly can in every minute that will, with the other things he says they should do, give them all the great prizes he lists at the end.

I am something of a fan of Kipling but I do, personally, consider "If" to be one of his less good pieces of work. It's never resonated with me, it seems to be asking too much and, to some extent, offering too little. Perhaps that says more about me than Rudyard.

The whole (beautiful) poem is about perseverance, striving, patience, and hope, particularly in the face of the cruelties that may beset one on one's adventure through life.

If you review each of the stanzas and verses, the poet (Kipling), is encouraging the listener (the author's "son," in this case) to just keep trying, no matter what; don't let the haters get to you, and stay true to yourself.

The unforgiving minute alludes to the relentless nature of time: it just keeps going, and we just keep getting older. As other answers have pointed out, time flows in only one direction, so you have to live with the consequences of your actions in the past, even if they're not "your fault" — time can be considered unforgiving in that sense.

In that minute, Kipling implores the reader to be mindful of this fact and pass every second meaningfully. Don't just goof off or pass your time in pursuit of idle pleasure, but run as hard and as far as you can. The "distance run" is a metaphorical distance and a metaphorical running, in reference to the exertion (running) and progress (distance) on personal goals. Kipling is calling upon the reader to devote every moment of their life to betterment and progress; in 1 minute, spend 60 seconds (that is, all of it) trying to be and do your best.