“There is a difference between Exercise and Training. Exercise is physical activity for its own sake, a workout done for the effect it produces today, during the workout or right after you're through. Training is physical activity done with a longer-term goal in mind, the constituent workouts of which are specifically designed to produce that goal. Training is how athletes prepare to win, and how all motivated people approach physical preparation.”
There are any number of books you can read on training, but Practical Programming is one of my all-time favorites. Why? Because instead of a science-to-the-gym approach, Rippetoe figured out what works in the real world, and then built systems around these insights. A couple decades later, science is catching up.
There are myriad lessons in the book, but most powerful of all is learning how complex your training truly needs to be, based on your experience. As a recreational skier and mountain biker, my training can be really simple and work wonderfully. As a more advanced climber, I need to look for marginal gains from ever more carefully considered training.
If you lift weights, run, climb, or move, this is an excellent guide to learning how we adapt to training.