Improve your body composition however you desire by training in a way that shapes your physique accordingly. Basically, this means that if you want to lose fat, you need to generate as much physiological stress as possible so that your body uses a significant amount of energy and produces a dramatic buildup of lactic acid in the shortest period of time.
The effect will be a lot of calorie burning, elevated fat burning, possibly muscle building, and a higher metabolism. Do this by lifting weights and doing high-intensity interval exercise. It’s hard and physiologically stressful, but highly effective.
If you want to build muscle, you again want to apply a lot of “stress” to the muscle and overload it with tension, or weight. The continual loading will cause the muscle to adapt and grow bigger over time. This can also have the effect of elevating the metabolism and burning fat, which is the reason weight training designed to build muscle is ideal for improving body composition since it gets rid of your flabby fat and increases your solid, compact muscle.
Aerobic exercise such as running is ALL ABOUT training the body to be as efficient as possible. The classic aerobic exercise modes that are inappropriately recommended for weight loss train the body to use the least amount of oxygen and energy to perform the greatest amount of work.
Nothing about aerobic exercise is in line with the goals of fat loss or muscle building. Because the body adapts so quickly to repetitive aerobic exercise, doing it for fat loss is choosing the least effective means to reach the end goal. Sure, endurance running and cycling burn more calories than sitting at your desk, but wouldn’t you rather use the right method to reach your goal?
Aerobic exercise is also the wrong mode to train if you are over 40 and your goal is mobility and longevity since it causes muscle loss over time and decreases power. The degradation of muscle mass and loss of explosive power that already comes with aging is accelerated when older folks do aerobic exercise, putting them at greater risk of falling and fracture.
Again, you could argue that doing endurance running is better for an older person than sitting on the couch all day, but if you actually want to live long, healthy, and be active, weight training will help you reach those goals.
To review, if you want to lose fat, do weight training and intervals. If you want to build muscle, overload the muscle with tension by lifting weights. If you want to be efficient but weak, do aerobic exercise. If you want to live long and healthy, do weight training that includes both heavy and powerful exercises.