The quote I used for the title of the post comes from this passage, where Murakami writes about a mental aspect of running marathons - having a mantra:
One runner told of a mantra his older brother, also a runner, had taught him which he's pondered ever since he began running. Here it is: Pain is inevitable. Suffering is optional. Say you're running and you start to think, Man this hurts, I can't take it anymore. The hurt part is an unavoidable reality, but whether or not you can stand any more is up to the runner himself. This pretty much sums up the most important aspect of marathon running.An excerpt from the book was published in a recent issue of The New Yorker, but unfortunately all that's online at this point is an abstract.
You can check out what some other runner/bloggers have to say about the book here and here.