The narrator, Adam, tells us the setting in the first sentence: he is on 'the bicycle' and is on 'Route 31'. Later in the chapter we find he's travelling from Monument, Massachusetts, to Rutterburg, Vermont.
It's not quite so straightforward, though. In the last chapter, when Adam reaches Rutterberg, he arrives at a hospital where he's questioned by a doctor. It becomes clear that Adam's journey has been in his imagination. He has been cycling, but only around the hospital grounds. So you could say that the novel's setting is the hospital, although this is only revealed retrospectively - that is, after we've read it.