If "If" is anything, it is a poem about endurance, about persevering, even in hard times. And we don't mean just literal endurance, as in the final stanza's discussion of distance running. There is also emotional, or figurative endurance—all that business about holding on, and finding the will to continue, even when one's body feels like it's about to break. In many ways, "If" is a poem about strength, and at times it seems like its underlying theme is "only the strong survive."
Line 5: The speaker talks about the importance of not getting tired of waiting. If nothing else, this line emphasizes the importance of patience, itself a form of endurance.
Lines 13-14: The word "bear" here means something like "endure" or "put up with," while the whole idea of having one's truth twisted is a metaphor for abuse and misuse of one's words.
Lines 15-16: "Watch" means literally watch, but it's also a metaphor for endurance or being able to suffer through the pain of watching what is near and dear be destroyed.
Lines 21-24: These lines anticipate that "distance run" business of the final stanza, but also stress the importance of the Will. The word "gone" in the description of nerves and sinews seems to be a metaphor for exhaustion, or fatigue.
Lines 29-30: You get only sixty seconds in a minute, so you better make the most of it. The phrase "sixty seconds' worth of distance run" is a metaphor for making the most of something or trying as hard as you can. In other words: carpe diem.