Since you already walk/bike much, and your body is already underweight, there aren't any real benefits from doing it IMO. Do you draw/paint? Doing sculptures? Play an instrument? Play games? or do have any other hobby that you would really like to do?
If you do, i would suggest that you don't do any of these until (per day) until you have done any other chore, thus your hobbies becomes an award for doing something. If you have a TV or computer or, turn them off. If you have to work on your computer, turn off your ADSL modem. If you need to work with data from the 'Net, then gather it and that, and then turn of your ADSL modem.
If you haven't found a hobby, do a search. Preferbly choosing one involving meeting real people, like a sport or something.
Good luck!