In truth, the first six months on Marquis’s trips are always harrowing. She describes it as “the washing machine”: endless agitation, physical pain, emotional pain, nonstop bargaining among opposing internal voices — the inner demons that whisper, Remember the delicious foam on the cafe latte? and the inner angels that reprimand, Coffee isn’t accessible now, so why talk about it? “You can’t move your hands, you can’t move your feet, you just want to die,” Marquis said. “You think about sleep all the time, because maybe sleep will set things straight.”

A few months into her journey, Marquis shot a video of herself in her sleeping bag. Like a hostage clutching a newspaper, she holds a thermometer that reads minus 20 Celsius. “I don’t sleep much these days. I do not know what time it is. Maybe midnight, or something like that?” In the next day’s video, she looks wrecked. The previous night a wind- and sandstorm ripped across the Mongolian plains. To keep the nylon of her tent from tearing, Marquis removed the metal poles holding it up. But she still feared the gales would blow away her gear, so she unzipped herself from her collapsed shelter and lay atop her pack, tent and cart.

Another night during those first months, while Marquis camped on a vast, overgrazed steppe that she describes as looking like an ugly golf course, she heard horses galloping toward her. The visitors turned out to be Mongol horsemen, all in traditional overcoat-like deels, making a vodka-fueled raid on her camp. After trying to steal her tent, they rode off. But for weeks, in the evenings, the men returned, treating Marquis, she said, as “the little entertainment.” To protect herself, she began waking before dawn, walking until midafternoon, then looking for a place to hide for the night — if possible, in a cement sewage pipe. “Everything is going on under those roads,” she said. “There is waste. There are dead sheep. But for me it was not a problem. I was safe.”

Eventually, however, Marquis passed out of Mongol territory. The washing-machine cycle ended. Her body changed, and her mind changed, too. Her senses sharpened to the point that she could smell shampoo on a tourist’s hair from a mile away. “One day you walk 12 hours, and you don’t feel pain,” Marquis said. The past and present telescope down to an all-consuming now. “There is no before or after. The intellect doesn’t drive you anymore. It doesn’t exist anymore. You become what nature needs you to be: this wild thing.”

As Francis Spufford writes in his history of British polar exploration, “I May Be Some Time,” for ages, men have wandered intentionally into extreme hardship, and they “are notoriously bad at saying why.” Marquis and her female peers — women who, say, walk across the Sahara alone with a camel or pull a 200-pound sled to the South Pole — don’t explain it much better. “People always ask, ‘Was it something in your childhood?’ ” says Felicity Aston, the first woman to ski solo across Antarctica. “I’ve thought about it endlessly: no.”

The rest of Marquis’s trip was not all Zen bliss. Seven months into the walk, she lost a molar. Her gum abscessed, and the attendant infection, which couldn’t be controlled with the antibiotics, started moving down her neck, and she had to be evacuated from Mongolia. Marquis returned to the precise G.P.S. coordinates she left and made it to China, where, one day, some children followed her. She sang with them and taught them how to set up her tent — and then they stole her BlackBerry. In Laos, drug dealers descended on Marquis’s camp one night, firing their automatic weapons into the air. Soon after that, Marquis contracted dengue fever. She tied her left leg to a tree so she wouldn’t wander off in her delirium and drown herself in a river.