Let me chime in with all the others who say NOW is the time to join a gym!
I was 46 years old and 257 pounds when I joined a gym for the first time in my life. It profoundly changed my life and is what made my weight loss and maintenance possible - you can click on the 'My Story' link in my signature if you want to read more.
My knees are terrible too, but a gym will have cardio equipment that works for us, like ellipticals and bikes. And strengthening the muscles that support the knee joint is the best thing you can do to preserve and protect bad knees.
Most gyms come with a session or two with a personal trainer to set up a workout routine. There are also some good resources listed in the 'stickys' in the Ladies Who Lift forum.
There's lots that someone of your size can do in a gym - in fact, the equipment is designed to fit large men, so it actually works pretty well for larger women (except for the whole height issue, but the seats on the machines are adjustable).
Fortunately, there aren't any expectations - the important thing is to move your body, elevate your heart rate, build a little muscle, and have fun. Go on and give it a try ... it might change your life too.