WHEN MY WIFE AND I DROPPED OUR PACKS at our first night's camp, below the Grand Canyon's South Rim, we were running on empty: water about gone, ditto our energy. We were on the New Hance Trail, a punishing, eight-mile unmaintained chute that plunges 4,600 feet to the Colorado River, about 20 miles east of the Bright Angel Trail. It's one of the fastest and most scenic of the South Rim's backcountry routes but also one of the toughest—the black-diamond express to the bottom.

Ginny and I had signed up for an eight-day guided trek into the canyon's stupendous eastern wilderness, and both of us were already grateful that we weren't going it alone. Because it's impossible to carry enough water for a multiday trek (every gallon weighs 8.3 pounds), you have to know where to find it—and now, in late May, with afternoon temperatures edging above 90 degrees, many of the inner canyon's hidden springs and seeps were rapidly vanishing. Hikers naive enough to try a rim-to-rim traverse during the 120-degree days of midsummer keep the Park Service's search-and-rescue teams busy. In July and August, fatalities from heatstroke aren't uncommon; a month after our trek, it would take the life of a 28-year-old British man.

We're both experienced in the wilderness, but not in the Grand Canyon. David Hogan, on the other hand, has been hiking and guiding the canyon's backcountry for nine years. A self-described "recovering New Yorker," he's easygoing, knowledgeable about the terrain and natural history, and a workhorse: He humped a 90-pound pack stuffed with food, fuel, cooking gear, water pump, first-aid kit, and sat phone while we carried about 35 pounds each. Our first camp was on an exposed, windswept terrace, about halfway to the Colorado. There wasn't a drop of water around except for Hogan's dwindling stash. He told us to relax while he trotted off in river sandals carrying two ten-liter bladders and a filter pump. The round-trip to a secret spring he knew took him two hours.

The next morning, I repeatedly fell behind while dawdling over some potsherd or fossil or rubbernecking at geological formations that make up what John Wesley Powell called "the most sublime spectacle on earth." To the men on Powell's expedition, and to other 19th-century explorers, the canyon's soaring geological features suggested temples of mythological deities such as Vishnu, Jupiter, and Venus. As I looked out at the rows of pinnacles and pagoda-shaped buttes, it was clear why the canyon has long been venerated as sacred.

It's said that every downward step in the canyon represents 60,000 years, so by the time we heard the roar of the Colorado, we had descended through about 1.8 billion years of geologic time. We joined the river at Hance Rapids, 76.8 river miles below the put-in at Lees Ferry. After a midday dip, we ate in a shady grove of tamarisk trees; until evening, we had the area pretty much to ourselves. Even though some 4.6 million people entered the park last year, it's estimated that fewer than 100,000 backpackers ventured below the rim.

Trail maps I'd studied suggested that once we reached the river, we'd be at leisure: strolling on beaches, promenading up side canyons, sipping cocktails proffered by generous rafters. Yes, we enjoyed all this. But as we hiked eastward on the Escalante Trail toward the Tanner Trail, our exit route, Hogan led us over rockslides of black schist that spilled into the river's turbulent, pea-green water. We clung to ledges the width of my boot. The footing was loose and scary, the drops long and precipitous. Plus I couldn't seem to consume enough water to keep my urine "clear and copious." To restore my wits, Hogan gave me packets of electrolyte-replacement crystals to mix with my water.

About two miles above Hance Rapids, we turned away from the river and headed into Seventy-Five-Mile Canyon. It would be two days before we rejoined the Colorado, and between us and fresh water lay a shadeless expanse called Furnace Flats. A group of four buddies we met had crossed it the day before. Having overestimated their hiking prowess, by nightfall they'd been forced to bivouac in a dry camp where their only liquid refreshment was 86 proof: liters of martinis and Scotch.

We, on the other hand, were guzzling slurpies the next afternoon. Hogan's fellow guide Shayne Hall moseyed into camp at noon, after powering down the Tanner Trail with a packful of frozen drinks and fresh supplies. After one big gulp, I developed a piercing headache—surely the first case of brain freeze at that particular spot.

Hall left us the next morning and planned to be on the rim before nightfall. He could complete the 14-mile hike in little more than six hours, whereas we'd be doing it in three days. On the way out, he stopped at Tanner Rapids to set up a tent for us. That wasn't necessary, of course, but it did lighten our load and reserve us a shaded campsite on a popular beach where rafters and overnight hikers usually snap up the prime spots. It was one more perk of having experience and foresight on our side.

The next day was blazing. I scooped out a seat beside the river and dipped into The Man Who Walked Through Time, Colin Fletcher's 1967 classic about his end-to-end canyon trek. Late that afternoon, before departing for our final night's camp, midway to the rim, we met four long-haul hikers from Chicago. One was on his 22nd canyon trek. Each journey, he told us, had been a "religious experience."

I wouldn't go that far about our trip. If I'd had any epiphany, it was in rediscovering, as Fletcher had, that the greatest reward of a hard walk in the wilderness is simply contentment. We were hooked on the canyon, and we'd picked up so many skills that we'd consider forgoing a guide next time. Then again, those slurpies were a godsend.