I get this all the time.
I really want to do martial arts but I need to get in shape first.
First let me say, it is important that you get a doctor's ok before you start so you don't do one front kick and keel over dead. That being said, it is not a requirement that you already be in shape! That's ridiculous! If you were already in perfect shape to do martial arts, my job becomes much less interesting :) You could just swing by. You'd show me your side kick. I'd suggest some adjustments to your form and that'd be it! The truth is no one starts their training in shape, even if they're in shape. Mmmmm..... what is that some kind of zen koan or something? Here's the deal. Let's say that you're a pretty good amateur triathlete and today is your first day in our beginner's class. Will you make it through the class without busting a lung? Yeah probably because you have good cardio. However you might not be able to kick higher than your shin because you have the flexibility of a death row inmate in leg irons. And you probably won't be able to punch effectively because your torso muscles haven't been developed yet. The point is that even if you're in shape, you're probably not in great martial arts shape yet.
Ok, but what if the most active thing I've done is cheer real loud while watching football on the tube? No prob here. Good martial arts classes are designed to take someone with zero training and build them from the ground up. This includes: