I have a female retriever thats 8 years old, I want her to live to be very old and stay healthy. I think shes a little out of shape, she has a very sore knee that she needed surgery on and the extra weight makes it sore . Shes on pain medication . How much exercise should I give her per day when exercise is a little hard for her sometimes because of her knee? Is there anything she can eat that will help her health? thanks