When Karl Bushby started walking north from Punta Arenas, Chile, pulling a handcart he'd nicknamed the Beast packed with 100 pounds of supplies, the 29-year-old British ex-paratrooper was aiming to do something no one had done before: circle the globe on foot. His intended route would take him through the Darién Gap, across the Mojave Desert, over the Bering Strait, through Mongolia and the Middle East, into the English Channel Tunnel, and finally back home to England, where he would be reunited with his parents and the son he'd left behind when the boy was just five.

Within hours, the Beast fell apart. Within a week, Bushby's toenails started popping off. He's survived near-starvation in Patagonia, a dust storm in Peru, brushes with crocodiles and narcoterrorists in Colombia, and a total of 71 days in jail in two different countries. Over 15 years, Bushby, now 44, has covered half the planet.

The son of a decorated Special Air Services officer, Bushby grew up feeling he had something to prove. In school, teachers humiliated him when he couldn't master his lessons – a diagnosis of dyslexia at 15 didn't remove the sting. The next year, he decided to try out for the army's elite Parachute Regiment. On his fifth attempt, he was given a "commanding officer's pass," meaning he was waved through more on effort and persistence than fitness for the job. The favor haunted him.

"Every time I stood in the mirror and donned my red beret, I was reminded of my shortcomings," Bushby says. "As a soldier, you're responsible for your brothers. If they're thinking there's a chance you don't deserve to be there, that's a burden almost impossible to bear."

Over the 12 years Bushby served in the military, his marriage fell apart, and he grappled with self-doubt. He soon realized that the only way to relieve it would be to accomplish something almost preposterous in scope, something only the hardiest, grittiest of men could pull off. So he started walking.

These days, Bushby is busy petitioning Russian authorities to grant him access to the eastern province of Yakutia, the farthest point he reached before visa snafus and suspicion of espionage stalled his trip. To conserve his last dollars, he flew to Melaque, Mexico, a town he'd walked through years ago. Each day, he checks his email, awaiting word that he can resume his journey. "It's quite a long walk," he says. "But I've made it my mission, and frankly, failure isn't an option."