I found the story and writing to be simplistic, but its message strong and enduring. The point is that we all imagine our lives working out without any flaws or setbacks, and yet, more often than not we have to deal with obstacles--tremendous in size and scope--that force us to adjust to our situations accordingly, whilst still living our lives. There's only two options when these obstacles come to fruition: deal with them or commit suicide. The second option isn't viable. So what does Tim (the protagonist) have to do? He has to adjust his life accordingly. Unfortunately, this means losing his job, his wife, his health, several of his fingers and toes, and any sort of purpose to life other than walking without end.
I would have appreciated more of a stronger middle and end to the novel, but they say that the "second book" is the most arduous. There was a sub-plot involving a murder suspect that could have been crucial to the underlying story, but nothing ever came out of it in the end. And of the four parts of the novel, the latter one was reminiscent of the myth of Sisyphus; such as Sisyphus was tasked with pushing a rock up a hill ad infinitum (devoid of any other purpose), so it went for Tim and his uncontrollable need to walk.
"One must imagine Sisyphus happy." - Albert Camus, Le Mythe de Sisyphe (1942).