There’s a specific moment a lot of runners over 40 remember clearly. You finish a run that should have been easy, something aches that didn’t ache last year, and a thought arrives uninvited: maybe I’m getting too old for this. It’s a persuasive thought, and it gets reinforced everywhere. A friend tells you their doctor said running is hard on the knees. Someone mentions that their brother had to give it up. The narrative is that running has a shelf life, your body is wearing out, and pain is the notification that your time is up. Here’s what’s usually happening instead.