Learn English – “time and time over” meaning

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.