Time.

Time is, at once, both an oppressive tyrant and a silent assassin. Every day we live with the knowledge that we have limited time, and yet it is very easy to fail to realize how much time has passed. This is so important when it comes to physical activity, and I believe a serious impediment to people when they start exercising again. Few people and few trainers are patient enough to realize that we need to rebuild ourselves from the ground up, getting the body balanced and THEN begin to strengthen in earnest. It might have been years, even decades since you pushed yourself and more than your muscles have atrophied. Chances are you have been holding your body incorrectly for a long time, neglecting vital muscles and those muscles need to be retrained, the connective tissues, the bones, they adhere to must all be gradually be brought back up to speed. This will help avoid injury and also help your body progress in a balanced fashion. Time can deceive you but patience will show you the truth.