Here, this picture is worth a hundred thousand words. We are looking at the cross-sections of three different sets of thighs.
Smack dab in the middle is the femur, then the muscle, and the fat surrounds the muscle. Look at the difference in the size of bones between the sedentary and active men! If you don't use it, you lose it. Our bones require forces to be put on them to stay strong. Our muscles require physical activity to maintain their mass.
The human body is a powerful machine. When you are young muscle is easily gained and maintained, but right around the time you hit your 30's it becomes harder to keep those gains.
All I can think of is how capable the person on the right was and how limited the person on the left had been. If you want a high quality of life into old age, you have to prioritize exercise and make movement a part of your lifestyle. The sedentary cross section scares me, and I want to do everything in my power to avoid that result. Don't become the sedentary cross section.