Gonna have knee surgery. I just lost 30lbs and I always eat right & walk. During the healing process, what can I do to keep the weight off while down?

Less calories. First, congratulations on the hard work you put in toward your 30 lbs weight loss! While you are recovering, it is understandable that it will be difficult to keep up your physical activity. You will likely need to cut down on your daily calorie consumption for a short period of time until you are able to walk again. You may want to cut it by no more than 10% for that short period.