When I was morbidly obese, I knew I should be exercising. But my back and knees would not let me. I could walk for short stretches, but I had to rest.
So, I decided to lose weight first, then exercise. Since the balanced carb and protein diet puts on muscle mass, I was actually ahead of the game by losing weight that way. When I started, I had 100 pounds of muscle pushing around 283 pounds. Now I have 130 pounds of muscle pushing 170 pounds around. Ask me how different I feel now!
At any rate, there are a couple of pieces of exercise gear that really help people with weak knees and backs. I work out every day for 20 minutes on my Healthrider. This exercises every muscle in your body except your neck, and it does it without hurting your back and knees. I am very fond of it.
I also do 2 sets of 12 back extensions on the Roman Chair pictured above. This has strengthened my back tremendously and chased away all my back problems.
I have just added a set of women's free weights, 1-2-3 lb sets, and I am adding them for resistance work. Resistance work gets the insulin into your cells better than any other type of exercise. Do cardio for your heart, but do resistance work for your insulin resistance.
I aim for 30 minutes exercise per day. That is good for about 60 points post prandial, and keeps my blood sugar under control the entire day.