WAS IT A VISION or a waking dream? I stood stranded in a ludicrously steep, boulder-strewn gully on a 3,000-foot rock pile called Mount Keen, in a state of physical distress that had moved beyond discomfort and infirmity, past complaint, through muttering and disorientation to a detached state of surrender. The sun beat down.

I was on the fifth day of a six-day coast-to-coast journey by mountain bike across the deceptively charming Scottish Highlands, a course originating at the Sea of the Hebrides, on the west coast, and wending its way over 220 miles of temperamental terrain toward the rumored North Sea. At the moment, I was no longer on my bike; it was on me, slung over my shoulders. Although I had, earlier in the week, grappled with a hint of hypothermia, a suggestion of sunstroke, numerous intimacies with mud, bramble, running water, and rock, and an ever-shifting array of orthopedic deficits, my most woeful moment was now upon me.

I had set out that morning after a sleepless night, having immoderately sampled the local offerings of haggis, black pudding, and other shadowy offal—treats chased down with a splash or three of the spirits for which the Highlands are justly famous. This day, touted as the most challenging of the trip, had started gently enough, climbing through a patch of remnant Caledonian forest, winding through open meadows infused with a dozen shades of green, following a disused "drove road," or cattle trail, along a meandering tributary of the River Dee, pausing for breath at the stone ruins of an ancient shepherd's hostel. But these splendors were all but lost on me. By the time I reached the base of Mount Keen, I was in the clutches of dehydration, legs wilted and head throbbing. My fellow riders, all a good deal more dedicated to the ideology of mountain biking than I—and therefore more prone to exult in suffering—had already crested the ridge above me and no longer offered even the remote companionability of blips on the bare hillside.

Our path, a crude mountainside incision with a grade of around 30 percent, made straight for the summit of this easternmost of Scotland's Munros, the 284 revered, pesky mountains that occupy the lordly heights between 3,000 and 4,400 feet. It was self-evidently unrideable and, in my current state, barely walkable, cobbled with heaps of stones that gave way underfoot with every step. I lurched unevenly along, the cleats of my bike shoes clacking forlornly. If Samuel Beckett had placed his existentially distressed characters on bikes, I thought, they might look like this.

It was at this point that our leader, John Fulton, materialized beside me. He was clad in Scotland's flag—a blue-and-white cycling jersey blazed with Saint Andrew's X-shaped cross—his bike raised above his shoulder like a trophy. Fulton bears a passing resemblance to his countryman Sean Connery and is the kind of outlandish physical specimen for which the region, whose traditional Highland games include log tossing and stone heaving, is renowned. A pioneer of off-road biking in Scotland, he quit his job as an engineer for Otis elevators nearly three decades ago to devote himself to being a guide. He remains, at 61, an extraordinarily powerful and sure rider. His small outfit, Wildcat Adventures, leads mountain-biking trips to far-flung destinations like Morocco and Mongolia, but the Scotland coast-to-coast is Fulton's classic.

"We had a lad from New York City on the ride a few years back," he said as I continued to struggle uphill. "It was a lovely day, like today, but as we were going up this very path, clouds started coming in. In no time, the skies were black. It started to rain. The wind was blowing brutally. By the time we got to where you and I are now, it was snowing." He paused and turned to survey the valley below. "This was late spring, mind you. This lad from New York had been overheated one moment, and now he was freezing cold. The path was so slick he kept stumbling as he tried to climb."

On cue, I slipped. Fulton chuckled, then went on with a grim matter-of-factness. "At one point the fellow looks at me with fear in his eyes and says, 'John, I think I'm going to die up here.' "

There was no punchline.