I did karate in grades 2-8, but I was never very keen on the fitness stuff and didn't push myself very hard. I enjoyed doing sports recreationally, e.g., soccer, volleyball, basketball. But then all through high school I would only have a PE class once a week and maybe do some cross country or cycling occasionally. So very little exercise. When I started P90X last summer, it was very much the discrete switch you describe. I was about to start working at a startup that summer, so I aligned the two to start the same day and committed to both.