Yoga is kicking my butt

My wife and I have been doing Bikram yoga classes. I've had 5 classes and every one has kicked my butt. We start by lying down on our towel and mat in the heated room. When it is time for the class to start, the instructor walks in and turns on the lights. The next 45 minutes takes the class through a series of standing postures, many of which require you to balance on one leg. I'm particularly bad at balancing on one leg, never mind doing so while kicking my other leg up over my head. Finally we go to a set of ground based postures punctuated by sit-ups. Many of the postures are familiar to me from my days as a runner, but they are all done a little bit differently in yoga.

From the beginning of class until it ends, I usually end up sweating out about 7-10 pounds of water. This tends to make my towel a little bit sodden and difficult to balance on. I guess I'd do better if I were going more often than once or twice a week, but right now that's all that fits into my schedule. I do feel like I'm getting a benefit from going. I just wish it would stop kicking my butt so much.