Is a gym membership wasteful?

Is a gym membership an unnecessary expense? This is one of my expenses that I feel is extravagant- only the rich go to gyms, right? I could just go for a run or walk at a nearby park or in my neighborhood. It would be free and I would also save the world my carbon emissions getting to the gym. I try to be frugal, but this is one thing I've struggled with. My main dilemma is that I really enjoy going to the gym. I love the classes and the instructors and the people. I am far more motivated to move at the gym than when left to my own accord. Many times I've gone for a "jog" and ended up walking and not going as far or as long as I planned. I realize that I'm not willing to part with my gym at this point so I'm trying to justify my spending. I try to think of it as an investment in my health that will pay off later on in health care and insurance costs. I did shop around and wait for a special membership price: a little over $20 a month. I figure that's not a bad price to pay for all types of classes from kickboxing and yoga to belly dance and aerobics and I even use the weight machines there. Now that I've attempted to rationalize my guilt for spending, I've decided one thing I can do to get my money's worth and drive less is to shower at the gym and head straight to school rather than driving home to shower. It takes more effort to pack a gym bag, but in return I get free shampoo, conditioner, body wash and hot water! I wonder how many showers a month add up to $20??