What does the poet imply when he urges one to "fill the unforgiving minute/ With sixty seconds’ worth of distance run?"
Time is a limited or finite resource. One minute comprises sixty seconds only; never more or never less. It’s “unforgiving” because once you’ve lost it, you’ve lost it forever. No matter how much you repent or fume at it, the lost moment would never return. Time moves on relentlessly, unsparingly.
Thus, the poet urges one to “fill the unforgiving minute / With sixty seconds’ worth of distance run.” This implies one must make the optimum use of the time available at hand. One must not take time for granted and become slack and negligent. Instead, one must regard time as one’s most valuable asset.
Even though we know a passed moment can’t be retrieved, yet we often trifle away time, spending it injudiciously. Quite often we simply waste it in frivolous activities or use it ineffectively. The poet urges us to value time, and utilize every minute of it in a way that we are able to make the most of it. This is what's implied in the quoted lines.