I adopted an elderly collie a few years back. She was 10 and had really bad arthritis and she was way overweight. So I gently took her for walks, gradually building up how far we went and I did watch how much she ate. I also took her swimming during the warm weather, which helped her joints and built up muscle. After about a year, she was much more active and had lost about 15 pounds. The vet wanted her to lose another 5 pounds, but... she was active and happy. Older animals, like people, sometimes put on a few pounds but that isn't necessarily a bad thing. I figured it was better at that point to let her enjoy her golden years then stress her out by trying to push her to lose those last few pounds. She lived to be 14 and except for the last few months, she was active.