MOVING FAST IN THE WILDERNESS, YOU MISS A LOT. This became obvious to me during a 100-mile backpacking trip in the High Sierra many years ago. I took the occasional rest day, lying around for hours and napping in the shade of a meadow's edge. Everything came alive to me in my stillness. I saw the tiny wildflowers I hadn't noticed while pounding down the trail. I saw high-altitude butterflies and two does and three fawns also waiting out the midday heat, unaware that I was watching.

On that same trip I met and chatted with a long-distance trail runner, a man who'd set out to run the entire 211-mile John Muir Trail in some ridiculously short time span--six days, I think.

He can't be noticing a thing beyond the dirt trail beneath his feet, I told myself. How sad; how beside the point. This maniacal runner, I decided, would completely miss the essential thing that wilderness offers humans: the mind's ability, when quiet, to discover the miracle of its own delight in nature. Ask yourself: Why is it that we never find a sunset ugly or a pristine green meadow tacky? In some mysterious way, the human brain and the natural world have arisen as paired phenomena, the one built to marvel at the other.

That was, as I said, a long time ago. I've since come to realize that I understood the issue only halfway. I see now that running inspires an atavistic appreciation of wilderness and our place in it--an appreciation we'd never find when sitting still.

"Natural Selection has designed us--from the structure of our brain cells to the structure of our big toe--for a career of seasonal journeys on foot," wrote Bruce Chatwin, the great English travel writer. Not just walking either. Humans, it turns out, are among the animal kingdom's great natural endurance runners, evolution having left us so hairless and riddled with sweat glands that we shed heat remarkably quickly and constantly. We're also blessed with enormous Achilles tendons that act like springs; another tendon keeps our heads from bobbling as we run.

All this allows conditioned humans to outdistance nearly every other beast on Earth, including hyenas, wolves, antelope, and even horses. The anthropological record is rich with stories of astounding endurance feats: Iroquois messengers trotting 240 miles in four days, for instance, and the Chasqui runners of the ancient Incan empire who guaranteed one-day delivery to anywhere within 150 miles.

We evolved to run, in other words. But not on pavement, which might explain why I always felt so much joy running trails. As a teenager, I escaped the confusion of adolescence during after-school jogs in the hills above Berkeley, California, encountering deer and the odd coyote. In college, on the woodland paths of upstate New York, I tried to feel at home in the Northeast's rolling, deciduous world, a place that never ceased to seem strange to a West Coast boy. During graduate school, I ran in the Santa Cruz Mountains, where I loved the sensation of moving from the dappled shadows of redwoods into open oak woodland with elevated views of Monterey Bay.

I enjoy walking, and of course I see more and think more at a slower pace--count more flower petals, as it were. But I feel vital and alive when I run. Not that it's easy, or easy on the body. By my late 20s, I had shin splints and patella-femoral syndrome, patellar tendonitis and plantar fasciitis. I bought thousand-dollar custom orthotics on the advice of a doctor who decided I had severe biomechanical problems in the form of weak arches and excessive pronation. Running-shop sales clerks would send me home with expensive "motion-control" running shoes: bootlike sneakers with extra padding under the heel. But the injuries only got worse, so I gave up. I figured I just wasn't meant to run.

In my mid-30s, I saw an article about how we'd evolved to run on the balls of our feet, not on our heels. The author, Christopher McDougall, has since expanded on this theory in his best-selling book, Born to Run. The gist goes like this: Despite decades of research and development by shoe companies, and despite shoes with ever more sophisticated support and cushioning, the rate of running injuries hasn't dropped a bit. Those who come from traditional running cultures, meanwhile--like the Kenyans and the Tarahumara Indians of northern Mexico, who run epic distances barefoot or wearing a simple flat sole--rarely get injured.

It turns out that thick-heeled shoes encourage us to strike first with our heels, then roll forward onto our toes. But no amount of padding can mitigate the shock that drives up through our skeletons with every footfall. After reading McDougall's article, I tried running barefoot around the block. I liked it. So I bought thin-soled running shoes without any padding, and that felt even better. Then I went out and found a stride coach, Brian MacKenzie, an ultramarathoner like the guy I'd seen years before in the High Sierra. MacKenzie watched me run, videotaped my stride, and gave me some drills, and soon my new foot strike and stride felt natural, strong.

Trail running had always been the point, for me, so I started up again and soon discovered that a natural gait on a dirt trail reduces impact even further. That's because every stride is different; your feet dance from rock to stick to soil. On flat pavement, you bang away at precisely the same physical imperfections over and over, until you get hurt. On a wandering footpath, impact varies from step to step and thus gets distributed throughout your body.

I had to run on pavement sometimes, of course. So, to make absolutely sure I didn't hurt myself, I would run the first mile or so barefoot, shoes in hand, which taught me two things. First, there's not nearly as much broken glass as you'd think, even in San Francisco, where I now live. Second, there's a wonderful self-limiting quality when you're barefoot. Almost instantly, you find ways to run softly and smoothly. I liked that feeling so much that I bought a pair of shoes called Vibram FiveFingers, which have articulated toes. They're like an artificial callus for people who like the idea of running barefoot but didn't grow up doing so and therefore don't have tough soles.

By the time I turned 40, I'd found that I could run 8, 10, even 12 miles up and down big hills in these thin-soled racing flats without any joint pain. More important, I'd grown closer to understanding why trail running had held such a grip on my imagination all those years, even when injury had kept me from doing it.

My favorite trail run these days begins on a paved path under big eucalyptus trees. I'm a creaky starter, so I never enjoy that first part. Then my route takes me across an asphalt parking lot, to an underpass below a loud four-lane highway, and into a place I love, a place where not too long ago I had my most inspiring moment as a trail runner: San Bruno Mountain State Park, one of America's largest urban open spaces and a huge and beautifully maintained quasi-wilderness of mountain lions and coyotes.

My hunger to run on trails instead of pavement had led me to scour maps for parks with intricate trail systems. And on San Bruno Mountain I found a loop that followed a narrow single track downhill for a mile. The real dance began there, among the willows, rushes, monkeyflower, and blackberry brambles that thrive in the freshwater seeping from the mountain's northern base. Embedded in the muddy patches were rocks and logs that told my feet where to go. Then the long climb began, slogging up the coastal scrub of the northwest flank into sage and coyote brush--and poison oak.

It was hard work, but thick fog obscured the view, so I felt long gone and all alone on that wild hillside. Wind drowned out everything but my own labored breath and the scuffling of my feet. Near the summit, the fog got wet and frigid. Visibility fell to maybe 100 yards, but I found pleasure in the privacy. The trail then became a dirt fire road, heading for several miles along the narrow southeast ridge, rising and falling on the mountain skyline. I looked down to my right on a curiously dry ecosystem: needlegrass and June grass and blue wild rye like thinning hair, waving gray in the wind on the southerly slope.

Then it happened. The marine-layer fog, a single white blanket off the ocean, poured up and over the ridge, keeping me inside the windy gloom--until suddenly I reached the blanket's edge and ran right out of it. In an instant, I blasted from a cold, misty day into a hot, clear one, as if through a door.

Alva Noe, the contemporary philosopher, has argued that consciousness isn't just a state of mind residing in the brain; it encompasses our whole body, our ongoing sensory interaction with the world. Trail running makes me think the same might be true of our mind's embrace of this earth: lungs alive, billowing in and out with the very substance of the sky, trading atmospheric gases with every tree and all the green grasses, our arms and legs alive in a million-year-old motion coded to make us feel fleet, and also to make us feel happy and right when flying along.

A still mind, for all its pleasures, cannot pop out of the side of a cloud like an airplane bursting into bright blue sky. A mind needs a body in motion to do that--to emerge into a view of the San Francisco Bay so expansive and sudden that it could be that of a pilot, speeding high above wind-ribboned water.