This is an easy question. Smoking. I quit recently, but starting smoking is easily the stupidest thing I have ever done. It has affected my fitness and it could still make me ill ten years from now. There is a saying, “its better to regret something you did do than something you didn’t” and I think there is some truth in that. However, smoking is such a bad idea, that I promise that nobody will ever regret that they didn’t do it.