The tone of the signage grows more frantic the farther you descend down South Kaibab Trail. At its starting point, atop the Grand Canyon’s South Rim, the trail is marked with notices reminding hikers that the path is without water and that “what goes down must go back up.” After a stretch of steep switchbacks drops you a quarter mile into the canyon’s rim, a sign advises not to try the walk to and from the Colorado River, which cuts across the canyon’s floor, in a single day. In four languages, it cautions against the steep, exposed 12-mile hike, but it’s the illustration — of a muscular blond hiker, burned red and projectile vomiting on all fours — that most succinctly communicates the message.

Most day hikers stop a mile and a half down at Cedar Ridge, a rocky panoramic lookout peppered with shrubs. From there, the canyon looks grand (as promised) and also sprawling and ancient, with its muted reds and greens. And yet from that height, the view still reads as more postcard than place. The canyon’s biggest draw and limitation are identical: its unintelligible dimensions. But on this October afternoon, a small white figure, growing in size, gave the eyes a much-needed sense of scale. As it worked its way up the winding dirt trail, it became clear that the white spot was a longhaired, pencil-thin man, running swiftly.

As he climbed the ridge’s left flank, a crowd of tourists began to point his way. I explained that he was Jim Walmsley, America’s best male ultra­runner. I told them he once ran from rim to rim to rim of the Grand Canyon — 42 miles with 12,000 feet of climbing — in under six hours. That he broke the course record at the Western States 100-Mile Endurance Run in back-to-back years and had run the fastest 50-miler in history just months before. The run we were witnessing that October day — more than half a marathon up and down the South Kaibab Trail — was, for Walmsley, just a casual tuneup.

Walmsley drew level with us, for a moment, before his hypnotic loping stride — metronomic and bounding, below a straight back — propelled him past in a disorienting flash. We watched him climb, at an unbreaking pace, onto the switchbacks and toward the trailhead. An old man turned to his wife and said: “Very cool! I saw two mule trains and an ultra­runner today.” I started the 1.5-mile hike back to the ridge. Walmsley was already around a corner and out of sight.