How is "steering" relevant in Heart of Darkness as a particular way of life in regards to Kurtz dying?

Marlow must steer the boat upriver to try to reach Kurtz, but he is unfamiliar with the river and is especially concerned about the hidden dangers below. He knows that any idea of control is just an illusion. Marlow learns the difference between substance and surface as well as the importance of hidden inner truth.

I had to keep guessing at the channel; I had to discern, mostly by inspiration, the signs of hidden banks; I watched for sunken stones; I was learning to clap my teeth smartly before my heart flew out . . .; I had to keep a look-out for the signs of dead wood we could cut up in the night for next day's steaming. When you have to attend to things of that sort, to the mere incidents of the surface, the reality—the reality, I tell you—fades. The inner truth is hidden—luckily, luckily.

When Marlow finally reaches Kurtz at his station deep in the interior, Kurtz is at death’s door, and Marlow decides that he must rescue him. The manager helps him get Kurtz carried onboard the boat that has brought Marlow up the river. Kurtz is delirious and makes little sense, but Marlow tries to understand his meaning. Marlow realizes that whatever control he had has now vanished. As soon as he saw the gruesome evidence of Kurtz’s deranged behavior—the decapitated heads on spikes—he realized how far-gone the other man was. Marlow hopes very much to bring Kurtz out of the jungle alive, in part so Kurtz can recover and present his own story, but he admits to himself that this outcome is highly unlikely. Any directing of the boat on the river, of Kurtz’s final days or hours of life, or even of his own fate are now out of his hands.

The brown current ran swiftly out of the heart of darkness, bearing us down towards the sea with twice the speed of our upward progress; and Kurtz's life was running swiftly too, ebbing, ebbing out of his heart into the sea of inexorable time.

Just before Kurtz dies, the boat breaks down, and Marlow must occupy himself with helping get it fixed. While the boat is repaired enough to continue the journey, Kurtz does not recover.
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