Fitness is boring and it isn't hard to fall of your routine if you are bored. When someone mentions the words self-defense or self-defense you immediately get "ewwww" or "yuck" because it conjures up a 1970's Bruce Lee wanna be. But self-defense is much more than that and has taken on new forms for younger generations. There are plenty of different options for all types of people.
Most of the self-defense enthusiasts are not your 20 year old studs that have something to prove. They are either small children working on coordination or older people who want to stay in shape. I have met the 20-30 year old competitive fighter but most of the people who engage in self-defense are not interested in this type of activity.
The benefits of self-defense are massive and range from the psychological to the physical. Once you have mastered basic techniques you will feel more confident about yourself. Your body language and movements change that signal to others that you are confident and worthy of respect. On the physical side your body will change as you engage in the coordinating and cardiovascular aspects of the sport.
To join or not to join is a personal choice. Fitness isn't always about running up and down the street or pushing massive weights to gain bulk. It should be balanced in its approach and sports and activities like self-defense is one way to do this. Search around and see if you like Karate, Kepo, Jiu-Jitsu, Aikido, Boxing, Wrestling, or anything else that suits your fancy.