Anatomy of a workout

Hi. I'm a sheep wanting to become a wolf (goldfish wanting to become a shark?). I follow everyone else's workouts but secretly wonder about the science behind them. Since I am in my forties and sometimes need to dummy down the workouts and don't always get to swim the same number of times a week, I was wondering how to structure my workouts to get the most out of them. Obviously I warm up. I can do 2-3500 yards (time permitting). Fins?, Pull, different strokes? What is the zen, ying yang etc...???? thanks.