I love the sport of boxing for getting into shape. Whether you are actually in the ring, into general fitness, or just learning basic self defense circuits offer an opportunity to lose fat and gain upper body strength. For a number of years I have engaged in martial arts and kickboxing. What I found is that the entire body must be developed to do well. A little more focus on your core and upper arms also encourages better performance.
Circuit training is high cardio workouts that help focus on the needed areas. If you do these on a regular basis you will lose weight and gain strength quickly. They are not easy though. You are likely to burn out fast so just add them to your workout when you can. Don't get lazy...just make it happen.