The intensity of your exercise is key, too. I know that as my cardiovascular stamina grew I was psyched that cardio was getting "easier." Yeah, it was "easier," but I wasn't getting the same benefits because I wasn't adapting my intensity level to my new fitness level -- and therefore, I wasn't burning as many calories while doing the same exercise. My cardio is exclusively the elliptical, and now I'm VERY conscious of keeping my body challenged by increasing the resistence every two weeks or so. So the next time you're exercising, ask yourself if your body is working as hard now as it was when you started. If the same old workout is easier now, CONGRATULATE YOURSELF, and then kick it up a notch or two!
Example: If your current exercise is to walk for say 20 minutes, that's great. But now you need to walk
faster, and maybe add on another 5 minutes.