Agreed. Getting back into shape or higher shape is often focusing on the part of your body current letting you down. A simple loop is you sprint, your legs gas out... Squats or other exercise. Legs strong now, you can't breath hard enough... More core... Now your core is stronger than your legs are. Legs again. Then consider stabilizers (I'm currently unable to squat more till I strengthen those).. etc etc. As I get older I'm learning more and more about stabilizers and tendons don't strengthen as fast as they used to.