Slinky
I practice shotokan karate. I will be brutally honest with you. In most (not all) karate dojos all the cardio you get is warmups before class that barely take 5 minutes to complete. Im not saying that you cant find one thats high intensity all the time. All we do in class is Kihon (basic techniques), followed by learning katas (forms of techniques strung together) and kumite (light to moderate sparring). Remember also, starting out at 10th kyu (most karate systems white belt), you will be learning techniques at a slow pace. You may get some more cardio once you recieve 3rd kyu-1st (the 3 brown belt ranks in most shotokan) kyu and shodan rank (shodan = first dan rank or black belt). Thats when most students do full contact sparring with pads. Karate on the other hand, does teach you balance, focus, and helps with flexibility.
If your interested in karate, kung fu, aikido, judo, jujitsu. I can help you find a school. Whatever style you want to do though, if you decide to, talk to me before you go in. There is a scourge of schools out now with less than honest business practices. They are out to make money, not to further the education of martial arts to their students. They have been dubbed "belt factories" or "McDojos" by the legitimate martial arts communities. Martial arts is one of my motivators to get the weight I've accumulated the last 2 years off.
for straight cardio, go with cardio kickboxing. Google search it and see what you come up with.