We need strength for the high notes (and quiet playing!), flexibility for getting around the horn in a hurry, and endurance, and each of these has its own practice requirements. For endurance, we need to play something that is low impact for a long period of time. Clarke studies are very good, the goal is to get the lips feeling a bit "tingly," resting a bit, then continuing. The tingling sensation comes from blood coursing through and gently expanding the capillaries. The better blood supply allows the waste products to leave the muscles more quickly, and more oxygen, sugars, etc. to reach the muscles. It is kind of like putting a bigger carburetor and exhaust system on a car, improving performance while using the same engine. Physical fitness is important, but strength training and endurance are two separate issues. Have fun!