one of life's cycles

You cut back your food to a "normal" portion size. Two hours later you're not only still hungry, but a little crabby as well. All the experts say you need to exercise as well. So you exercise, but not too much too soon, because you need to build up to it. And yet, the next day every joint in your body constantly exudes reminders that you are not as young as you used to be. SO, you have the crazy circle - eat what you want, do what you want and feel somewhat okay; OR eat what you should eat, do what you ought to do, be "healthy" but totally miserable. Am I missing something somewhere?