Yeh doesn't take much to let yourself go, you have to keep relativly active. Jogging just doesn't cut it. You have to actually work your muscles like doing a bit of sprinting at the end of your jog, to push your muscles.
I used to do next to nothing except a little roller blading after being extremely active. I still felt really fit and still looked fit. Then I did some social sport for 30 mins a week and my whole body would be sore for a couple of days after that. Its not cool
.
Now I do more exercise so I am not sore afterwards. I am only 23 too heh.