I'm a 20+ year veteran of martial arts. Here's the deal if you are looking to stay fit and defend yourself. (and I'll try to answer before all the video-game grandmasters comment)
Kick boxing and Muay Thai are geared for the ring, not self defense. The last thing you want to do in a SD situation is square-off with your opponent. Also, in Thailand, the average Muay Thai fighter has a ring-life of 1.5 years. That should tell you something about the training and how good it is for your body. A previous poster mentioned TKD. Tae Kwon Do is an absolute joke in any arena you put it in.
Considering you dont want to spend a decade or so learning Karate (it actually takes longer to really undertand) Krav Maga could be a good option. The one downside of Krav Maga is, because it really doesnt have a formalized system, instruction varies tremendously from teacher to teacher. Find one with a decent reputation and give it a shot.
Personally, I'm not down with anything MMA related and quite frankly, MMA & BJJ are horrible in a self defense situation (unless of course there are rules the next time youre attacked). What are you going to do, go to the ground in a bar while the guy's friend wastes you with a bottle? Also, unless you like tattoos, shaved heads and your family bowls a lot, MMA is a little WT.
"But resist we much; we must; and we will much; about that be committed."
~Rev. Al Sharpton
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