My headlamp illuminated yet another snow patch, this one close to 30 feet across, looming out of the shadows to completely swallow the trail. Even on a path this narrow, contouring along the side of the valley at night should have been easy, but these intermittent snow fields were killing me. Ollie, my hiking partner, barely slowed, stamping footprints into the crust, but I hesitated. I couldn’t trust my malfunctioning left leg to negotiate the steep and icy traverse, certainly not with a runout that disappeared into darkness. I grunted, turned, and began scrabbling up the muddy incline above the trail, my useless limb trailing behind me. By the time I’d climbed high enough to skirt the snow patch and slide back down to the path, I was filthy, exhausted, and frustrated. In the 14 hours since breaking camp, we’d averaged one mile per hour, taking our grand total over the last two days to 26. Out of 501.

　　I’d wanted to hike California’s Sierra Nevada for over a decade, my greedy eye fixed on a classic north-south traverse along the Pacific Crest Trail. Geologically, the range runs from Lake Almanor in the north to Tehachapi Pass, near the Southern California desert town of Mojave, but I was only really interested in the highest, most spectacular section. After playing around with a PCT distance calculator the previous year, I’d discovered that the route between Donner Pass and Walker Pass never fell below 5,000 feet and ran for exactly 501 miles, one mile over the minimum requirement for a permit. I loved the thought of breaking 500; It was 2022, I would turn 50 the following spring, and the challenge of hiking ten times my age intrigued me. Everything had fallen nicely into place. Except for one thing.

　　In 2019, I was diagnosed with multiple sclerosis (MS), a neurological disease in which the body’s immune system attacks the protective myelin sheaths around the nerves in the brain and spine, exposing them to damage and resulting in a poorly functioning central nervous system. Effectively, the signals sent by the brain to the rest of the body are interrupted.

　　The first evidence of the disease cropped up in 2017, when my legs started going through periods of uncontrollable twitching, contracting violently every couple of minutes. It was impossible to sleep during these attacks, or drive, or sit watching television. I was referred to a neurologist, who diagnosed me with restless-leg syndrome (RLS), and I now take a drug called pramipexole three times a day to prevent the spasms from driving me insane.

　　Then, in 2018, my left foot began to drag when it got tired. This would have been problematic for anyone, but hiking has been my primary obsession for over three decades, so the inconvenience was tinged with panic. I was born in the UK but moved to my adopted home of Sydney, Australia, in 2009. I’ve walked all over the world, from the Andes to the Alps to the Himalayas. Then there was trail running. I’d won an ultra in my time, and was only the second recorded person to run the full 43-mile circumference of Easter Island. I’d just started training for the 100-kilometer Ultra Trail Australia when the dragging began, and that was the end of that.

　　I like to think I’m a level-headed individual. I didn’t rant or damn God or even cry when, via an MRI scan and spinal tap, my neurologist diagnosed me with MS. What I did was ignore it as best I could and carry on with my life. That first summer, as planned, I walked the UK Coast to Coast path, the Haute Route in Switzerland, and Corsica’s GR20 back-to-back, a total of 442 miles. I called it the European Triple Crown.

　　Such feats meant I could pretend this slow-motion car crash wasn’t happening. The disease became my secret shame, known to only a handful of people: my doctors, my wife, and select colleagues. I felt embarrassed that my body was failing to do its job, as if it were somehow a moral weakness. To friends who caught me limping, I spun a lie about a rolled ankle. I couldn’t face their sympathy, and that unintentional look of pity behind the eyes. As far as my family was concerned, I didn’t want to worry them. Even today, five years after my initial diagnosis, I haven’t told my parents.

　　As I began planning the Sierra traverse in late 2022, my condition deteriorated rapidly. I went from breezing through multiday hikes to being unable to walk for half an hour without my left leg dragging behind me, like a sandbag attached to my knee.

　　There was no question of postponing the hike, though. The most common form of MS is called relapsing remitting, sufferers of which can sometimes live a relatively normal life between episodes of catastrophic disability. The other form, primary progressive MS, with which I am blessed, steadily worsens over time, a constant downhill slide into neurological oblivion. I can’t afford to delay anything today that I may not be able to do tomorrow.

　　When I’d revealed my plans to my neurologist, a man of measured understatement, his eyebrows had risen a good inch. He’d recovered well, though, suggesting a series of exercises to strengthen my hip flexors—the muscles that bring our legs up when we walk. When these are malfunctioning, I can trip over the tiniest crack in the pavement. Something like a fallen log presents a major impediment, as I have to use my arms to physically lift my legs over it. Nevertheless, I’d stubbornly declared that whether it took me eight, ten, or twelve hours per day, I would go the distance. Little did I know that twelve hours would be my minimum daily slog.