Thank her or blame her, Cheryl Strayed and her mega-popular book Wild have turned thru-hiking into a mainstream national pastime, with trails like the Pacific Crest, Appalachian, and Continental Divide getting exponentially more popular—and crowded.

But there’s one great U.S. thru-hike that’s been mostly spared the influx. Few people have heard of, much less attempted, the Great Western Loop, a 7,000-mile route that patches together portions of the Pacific Crest, Pacific Northwest, Continental Divide, Grand Enchantment, and Arizona trails, with sections of trail-less walking through the Sonoran and Mojave deserts. No official designation exists for the Great Western, which traverses some of the West’s most beautiful, rugged, and remote terrain, dipping into nine states, 12 national parks and more than 75 wilderness areas.

Only one person has ever completed the epic—professional backpacker and guide Andrew Skurka, who started the Loop in California and ended in Arizona. Jeff Garmire, a 27-year-old from Vancouver, Washington, plans to be the second. And judging by the more than 5,100 miles he’s already completed, he’s got a pretty decent chance at success.

Although the Great Western Loop reaches some of the highest elevations in the U.S.—Garmire chose to summit 14,505-foot Mount Whitney—it’s not the terrain that makes it so daunting. “The biggest challenge of the GWL is a mathematical one,” Skurka told me this fall. The Sierra and San Juan mountain ranges stay buried in snow much of the year, raising the risk of blizzards and avalanches, and making long-distance backpacking gear untenable. “To do it in one shot, you have to complete about 4,600 miles of trail in about four months.” In other words, you have to hike the entire distance of the Appalachian Trail twice in less than the length of a single season.