Sitting and being sedentary could be making you achy. If you have aches and pains your body could be telling you to get out and move! Gentle movement can get you feeling better.
Weight baring exercises help with keeping your bones strong as you grow older. Without exercise, your tendons and ligaments can become shorter, leaving you vulnerable to injury.
Moving your body will create the willingness to move your body even more. Try standing up every hour while you are awake during the day. Tiny changes are what will make a huge difference on your path to habit change.