TKD is fine if you're just doing it to get active...
but if you actually want to have an effective martial art for self defense, you'd be much better served studying Muay Thai.
Both are going to give you the workout you want, but why not develop a more functional skill since you're putting all that work in anyways?
If you have an MMA club in your area, better yet. There you'll work some muay thai, some wrestling, some jiujitsu...it keeps it interesting and you are a heck of a lot better off from a self-defense standpoint as your skills are well-rounded.
Belts are seriously overrated.