Diet is key. Those "back-to-back" people may be paying dues for their pint of ice cream. Weight training is important. Muscle is more metabolically active than fat. You can add more muscle to your legs doing squats than doing spin classes. But - both are important.

People who hog bikes when others are turned away REALLY burn me up. Our gym used to make people sign up 10 minutes in advance. The back-to-backers would just get off their bikes 10 minutes before the first class ended to sign up for the second class. That's just wrong, in my book.

But nobody reads "my book"