Sometimes it just becomes so technical that you forget what you're doing. If you start thinking about how you come down the stairs and think about how each muscle is working, you can't go down the stairs. Anyway, I'm a person who overthinks and overanalyzes everything, so if you give me one thought, it creates a lot more.
I work out a lot, but it changes day to day. I always start out with some cardio - either a jog, a bike ride, or footwork drills designed specifically for tennis movement. Then I do weights, but I switch the days: one day it's upper body, the next day it's lower body. Then I do stomach and back pretty much every day.