The most expensive mistake I ever made in fitness cost me exactly three hundred dollars, and I didn’t even feel it happen. It was early January, years ago, and I ordered a rower at full price because a New Year’s crowd had cleaned out every other machine in my size range. Eleven months later, that same model sat on the same website with a Black Friday banner across it — three hundred dollars cheaper. I stood in my kitchen staring at my phone like the screen itself had pickpocketed me. That was the day retail lost its magic for me,…