The naturalist Robert Lloyd Praeger, who tramped all over Ireland early in the last century and recounted what he saw in The Way That I Went, says in his introduction that even a bicycle was too fast for careful observation. Too fast for thinking, too. One of the chapters in Rebecca Solnit's Wanderlust: A History of Walking is called "The Mind at Three Miles an Hour." That's about the pace at which the brain works, no doubt because for most of our evolutionary history the brain was foot-powered. The Greek philosophers were not called peripatetics for nothing. No one ever had a great thought while ripping down the freeway at sixty miles an hour. Rousseau tells us in the Confessions that his mind only works with his legs: "When I stop walking, I cease to think."
Walking is the one thing that connects us to the deep past. Food, drink, clothing, shelter, sex, childbirth have all been transformed by technology, mostly for the better. But when we walk, we might as well be on the savannas of East Africa two million years ago. Wait, what am I saying? Of course walking has been transformed by technology. Most of the people I pass when I'm walking have on ear phones. They might as well be on a treadmill at home. In fact, I would guess that most of the walking Americans do is on treadmills. Curious, since service on treadmills has long been a form of indentured servitude. We enslave ourselves to our machines.
I'd rather think of walking as a spiritual activity. It has nothing to do with keeping the body fit, although that may be a convenient side effect. Walking is a time for the natural rhythms of the organism to assert themselves -- limbs, breath, heartbeat, thought -- a smoothly functioning unity honed by natural selection at a time when we were still a part of the natural world, not masters of it.