This isn't really from the show the biggest loser, but you should just try different athletic activities until you find something you like more then the others. Some people like cardio workout tapes, others swimming, volleyball, cycling, bungee jumping, hiking in the wilderness, or just walking. It just depends on you. Are you a team oriented person, a person who wants something very social, or are you a loner dude who is interested in eventually being able to ride your bike 100 miles up the mountain nobody else wants to go up. You could look for groups of people who are interested and meet up once a week to do stuff like that, or join a karate class and kick someone's butt.
You know?
What kind of challenge are you looking for? You could join a competitive sporting environment....like intermural sports in college or the entry level of some sports association.