I think most fitness experts would tell you to increase activity progressively, not to go from completely inactive to "gym rat" in one day. The older you are, t

I think most fitness experts would tell you to increase activity progressively, not to go from completely inactive to "gym rat" in one day. The older you are, the more important this is. The smartest way to do it is probably to increase both frequency and intensity progressively, which means your body can make small adaptations over time without putting undue stress on the joints, tendons, bones, cardio-vascular system etc.