yea, thats a pretty bad run for battery.
are you doing anything else with our phone when youre at the gym? facebooking? angry birds?
the only times i experience battery drain that fast is when my girlfriend is playing fruit ninja or something. otherwise, when im streaming music at work i can get at least 10 hours before i get a battery alert.
i usually suspect background data to be the cause for quick battery drains. but who really knows?
the t mobile comet gets pretty good battery life, about 2 days with moderate use. if you want long battery life, dont get a smart phone. my old motorolla v60 got about 3-4 days.