If I may make a suggestion here. If you suffer from bad knees, then you can do sitting exercises. Even something as simple as doing some arm exercises with a soup can in each hand is an excellent start. You can even do it while you are watching TV! It will work your upper body muscles, it will burn calories, it will increase your heart rate...and it is a very good, very gentle way for someone over 50 and overweight to start exercising without risking your health. Even if you only can do it for 5 minutes at a time, just do it!! You can do it 2 or 3 times during the day to start with.
Another thing you can try to do is walk around your house more than you do now. Make a few extra trips up and down the hall, every extra step you add to your day is a step in the right direction!!
There is nothing wrong with starting slow and easy....as long as you start. You will notice quite quickly how your stamina improves, and those 5 minute sessions turn into 7 minutes and then 10 minutes!!