I have discovered the joys of growing my own lettuce (in pots, since I rent) and can have a salad ANY TIME I want! It's great to add a little tuna or turkey and some vinagrette-type dressing. I always keep baby carrots on hand, and a bag of small apples and peanut butter.
I have bad knees, too, and fibromyalgia, and arthritis, but I force myself to walk, not that that's your answer. The YMCA has some great programs, especially water aerobics and low-impact work-outs specially designed for seniors and those of us with physical challenges. There are a lot of financial programs for people who may not be able to afford the classes or have certain kinds of insurances. Sometimes you can even get FREE membership (like my Mom who has Humana). Some simple stretching and flexing excercises keep your flexibility up, too. I also do light weight-lifting for strengthening those pesky back and neck muscles that insist on providing me with a majority of my pain.
I LOVE my breakfast! Eggs, cottage cheese, plain yogurt with fruit mixed in (I make my own and have found it's a lot less sour than store-bought!), fruit like apples with peanut butter, oatmeal, a little meat left over from last night's dinner. It's okay to have dinner for breakfast, that's usually where you get your protein for the day. If I eat a nice hearty breakfast (still low-carb), I have great numbers through the day and TONS more energy. That means I want to move more, that means I want to excercise more! I know how hard it was for me to get started on the work-outs, but I found after a couple of weeks of FORCING myself to walk that little bit extra my pain decreased significantly. Ask your doc what you are ALLOWED to do first!
Good luck! Keep us posted on your progress!