I was once a runner in my thirties. I trained for and completed a half marathon — a huge milestone in my life. Shortly after that race, I went for my 40-year-old annual checkup. The doctor smiled and said, “Welcome to the aging club! Now that you’re in your forties, you’ve got to take good care of yourself. If you don’t want knee injuries, you’d better stop running.” And guess what? I listened. I stopped running. Part of it was fear — fear of injury. Another part was practica