The best thing about Krav Maga is that I don't even notice I'm getting a workout until the sweat starts to drip. My mind is so engaged in focusing on learning the techniques, as opposed to grinding away on a treadmill, that it feels worth getting tired.
Frankly, I am in awful shape but Chris always lets me take as many breaks as I need and I've never been made to feel inadequate for usually being in poorer shape than the rest of the class. He's motivational without being flat out mean. Highly recommended.