With all the wonderful fruit and fish you'll be eating and all the swimming, snorkeling, walking, hiking, punta dancing, etc. you might even lose weight. I wasn't my usual couch potato self when I was there, so I lost a few pounds! Belize it. Debbie
Mostly there's weight lifting stuff; universal; at one point there was a group of arobicisers using a video .... it's open aired and located at the north end just below the boca del rio ferry and adjacent to a part (which was pretty badly hit by Keith); not sure how the park is right now, but you can still pump some iron and see the Sea! But, like it was said before, with a bit of walking, biking just getting from here to there coupled with lots of old fashioned sweat from the tropics, you'll probably feel fine.
Thanks everyone for your responses. I've been working out almost everyday to get back in shape so I can wear a swimming suit without being laughed at in AC and I just want the security of knowing there's a place I can go to work off those Belikens. Oscar's it is.