A short, quiet poem about a habit: stepping out the back door each night to walk through the fields. The speaker has recorded the walk, and the title, "Documentary," suggests it is a kind of film. That mile or two now "lives" in the camera, as if the walk has moved somewhere else and gone on breathing there.
The last two lines are the trickiest. "Taking as old wool / the whole story" seems to say the speaker accepts the whole thing the way you might accept a worn, familiar wool garment: comfortable, a little rough, already shaped by use. Judging by the imagery, the speaker isn't trying to make the walk dramatic. They just take in the whole story, plainly.
The mood is calm, private and a little tender. There is a gentle wonder at how an ordinary routine can be kept, and perhaps a quiet question about what is saved when we film something we have simply lived.