Do some walking every day if you are able. If you are not planning to do some walking in port, and on the days you are just cruising, take some laps on the promenade or sun deck. You can enjoy the fresh air, meet lots of people doing the same thing and have room for dessert.
This is my first time in this forum as well as my first time to Cruise. My trip starts on September 2007, at DineyWorld and then aboard the Disney Wonder. All the information I've read on the posts are great: keep it coming. I can use all of it since this is all new to me.