my mother in law started walking i'd say about 6 months ago. she lost about 40 lbs (she was about a deuce to begin with). i keep telling her the weight loss is gonna stop soon like some kind of a tool, but she's going strong. i'm not trying out for the olympics anytime soon, but would like to be a little more active.
Based on what you have said, it sounds like walking would be perfect for you. You could always move into more strenuous activities later on, but you may find that true fitness walking is not a "walk in the park" so to speak.
If nothing else, you cannot continue to be shown up by your mother in law.