Explain Magwitch’s belief that life is a river. Is the metaphor appropriate?

The river is consistently used throughout the novel, mostly in regards to Pip, but for Magwitch it becomes the end of the ride. He's taken the journey on water more than once. In deference to his own age, his feelings of exhaustion, or just content with the relationship he's forged with Pip, Magwitch is ready for the running to end.