If the gain in weight is excessive, I would suggest you to get tests done for diabetes (Blood sugar levels), hyper tension & thyroid function. And let me know what the results say for a better treatment. As it is, with age metabolism of the body starts decreasing, so one needs to bring required changes in their lifestyle too. Add a lot of fibre, leafy vegetables and fruits in your diet. Try to stay away from foods that have high sugar content. Also make some kind of exercise a part of your routine. Walking is a good option considering your age and health status. Don t let the knee pain stop u from walking as it would only make things worse with time. You can also go in for HRT(hormone replacement therapy).