Exercises for the back

Patient: I’ve had 4 back surgeries. My lower lumbar is fused, with bone graft and titanium plate and screws. I walk 1 mile 5 days a wk., with a walker, sitting at intervals, do 45 min. of “Sit and Be Fit” exercises (one with 2 lb. weights) and 15 min. of stretching floor exercises. When standing or walking, I need to lean on something, otherwise my body is soon parallel to the floor. My back hurts only when I stand or walk. My surgeon says my upper spine is ok. What exercises, if any, would help strengthen my back and help me stand straighter?I have a dead nerve to the left heel, from the first surgery, bone on bone in my knees and the start of osteopenia.