Well building muscle, does burn fat; so you can get into better shape by just lifting. However, while you will look good physically; you wont be in very good aerobic shape which is important to heart health. I figure, if your in the gym, you might as well hit everything and improve your total health all around. If you find the elliptical, bikes etc boring(which I do to)try incorporating cardio circuit workouts or HIIT. At the end of the day though, diet is the major mechanism for weight lost. We're not sure if we're doing TTR this year, but we are focused on losing weight--so besides hitting the gym 5x a week, we have focused on diet. So one month into this, I've(Andrew) have dropped 21lbs and she's dropped 13.