Face up to the fact that GRAVITY is working on pulling your body down every moment of every day. If you don't work to counteract the effects you will sag like an old beach ball.
Keeping you muscles strong and your bones solid and supportive will do a lot to keep your body erect and your back straight.
Regular stress on the frame and muscle will accomplish a lot for keeping you feeling and looking younger than your years.
Pick up something heavy!
If your work out consists of a tread mill or stationary bike and no lifting or weight machines you are missing the best you can do for your overall conditioning.
Why spend a half hour pedaling to nowhere?
Lift some weights!