In my mind, I could still run up mountains where others had to walk. No matter how temporarily slothful I became, I always had enough residual fitness to be up for any adventure with anyone. Alas, my self-image was seriously out of date, and the strength of the delusion only made greater the shock that followed.

There is a hill called Mt Iron on the outskirts of our town, shaped like a Sphinx and clearly the work of Ice Age glaciers. In other places you might call it a mountain, but in Wanaka, on the edge of the Southern Alps, it’s merely a hill, its zigzagging trails a jogging loop for townsfolk of all ages. In the past, I could easily run up and down this hill, twice in a session.

But this time it was different. As soon as I reached the bottom hairpin bend of the climb, I knew something was wrong. My heartbeat had a subwoofer quality and the tick of a runaway metronome. I couldn’t get enough air and my legs felt as though they were not my own. By the time I climbed to the third bend, my body refused to go on. I stopped, bent double, hands on knees, feeling like I might faint, sucking air like a man drowning.

“You okay, dear?” A woman, her voice tinged with concern, guided me to the track’s verge. “There, sit down and have a spell,” she said. “It’s quite steep through here, isn’t it?”

Permed platinum hair under a visor cap, jogging shorts, ruddy face and a friendly smile. She was 20 years my senior, perhaps more, and had been jogging right behind, though didn’t seem to be exhibiting an ounce of exertion.

“I’ll check on you again on my way down,” grandma said. “You take it easy now, won’t you?” Then she was off, trotting lightly up the long hill, looking quite at ease, as if she could go on forever.

I buried my face in my hands. I wasn’t just out of shape. I was heart attack material.

I walked the rest of the hill while joggers and runners passed me in both directions, grandma among them, hotfooting down the track like someone half her age. Down from the summit I too broke into a cautious trot, and soon found an easy gravity-assisted pace. Suddenly, it felt good to be alive, to just run with next to no effort. I had forgotten the feeling, but I would never forget it again, I vowed. I would get fit, and stay fit. During the descent, I was buoyed by the idea, and the sensations of running—the cooling breeze against the skin, the views, the whiffs of sun-dried wild thyme, the mesmeric rhythm of my footfalls. The runner’s high.