Do 100 push-ups, 100 sit-ups, 50 leg lifts every morning. What! Try do sets of 20 each, can't do 20 push-ups do as many as you can then do knee push-ups. By the way push-ups really add to core strength because you have your abs engaged much like the plank exercises. If this workout is too hard do it every other day. I'm 61 and I do it took me about 3 months to get used to it. I rest about minute between sets, sometimes two minutes I do my workout while watching the morning news so the time flies by. Good luck your back will feel better with exercise mine certainly did.