I actually cut back on tkd recently by 1 day a week because it was adversely affecting my fitness. Im a cardio junkie, and unfortunately (from a strictly fitness angle) a martial arts class will usually have 'down' time where the instructor is demonstrating techs, or you are working on wrist locks, grabs etc. Tkd is a great start for someone who does no exercise and wants a good starting point, but if exercise is the main goal then running, aerobics, circuit training, squash or many other activities are far better suited.