I have walked through many lives, some of them my own, and I am not who I was, (1-3)
Walking through one life takes a while, let alone many. We are introduced to the speaker in these opening lines that challenge the idea of time in a big way. According the speaker, life isn't short, and it might involve multiple trips around the Ferris wheel. If that's not enough, the speaker's lived so much that he doesn't even identify with who he used to be. That's a lot of living, and a lot of time.
When I look behind, as I am compelled to look before I can gather strength to proceed on my journey, (7-10)
The speaker feels like he has to look backwards in time before he can move forward. Somehow, this is related to growing stronger, but I don't think he's trying to recall cardio kickboxing classes. What kind of strength is he referring to?
I see the milestones dwindling toward the horizon (11-12)
This image, milestones shrinking over a great expanse, could be seen as an analogy for time passing over many years. We know that traveling for a long distance takes time, so Kunitz is ramping up that idea, showing us just how long the speaker's journey has been. Keep an eye out for more stone imagery as the poem progresses.