Running doesn’t wreck your knees. It strengthens them.

From the article:

[F]or a 2019 study, he and his orthopedic colleagues recruited 82 middle-aged, first-time racers who had signed up for the 2017 London Marathon. Few had done much, if any, previous running and none felt knee pain. The researchers scanned the runners’ knees before they began a structured, four-month training program and, again, two weeks after most completed the marathon. (Eleven dropped out during training.)

Interestingly, although none of the runners reported knee problems at the start, most of their first scans showed signs of incipient joint injuries, including cartilage tears and bone-marrow lesions, any of which could be early steps toward arthritis.

But this simmering damage had been partially reversed by training and racing. Two weeks after their first marathon, most of the runners’ existing bone marrow lesions had shrunk, as had much of their frayed cartilage.

But some runners showed symptoms of new, if slight, damage in the bones and cartilage right around their kneecaps, a part of the joint that absorbs much of the pounding from running. [...]

The scientist-surgeons helped settle that concern, though, with a follow-up study, published in 2020, during which they scanned the runners’ knees again, six months after their race. Most of them still ran, although with reduced mileage.

The new scans showed their knees were healthier now than in the weeks right after the marathon. Many of the lesions and tears that had begun shrinking during training were smaller and the fresh damage seen around some kneecaps had largely dissipated, with few remaining signs of lesions and tears.