A gruelling marathon and cycle ride through Switzerland’s mountains is not the average way to celebrate your 61st birthday. “I’d sworn I would never do an event like this – it seemed so long and brutal, with so much suffering,” says Sunny McKee. But here she was, competing in the formidable Ironman Championships. It involved cycling 180 kilometres (112 miles), combined with a marathon and a four-kilometre (2.5-mile) swim. All in a single day.

McKee was no stranger to intense exercise; she had been competing in regular, shorter, triathlons for 30 years. But it was only when she visited the championships in 2007 that she’d decided to take on an “ultra-endurance” event. There, she saw a double amputee manage to finish the race, despite having to stop every 20 minutes to empty his prosthetic legs of the sweat. “I decided that if someone like that could do it, then someone like me could too.”

Now 65, she is hooked, and competed in her latest Ironman event this October, with no ill effects. “I can run rings around my children and grandchildren,” she says.

A few decades ago, facing that kind of intense challenge after middle age might have seemed unthinkable. But over the past few years, it has become surprisingly common to find pensioners performing in ultra-endurance events like Ironman triathlons. A few are even running 20 consecutive marathons and cycling the entire breadth of America – across mountain and desert – on almost no sleep. So how do they do it? And what do these amazing feats tell us about the limits of the ageing body?