Are you using the Power Yoga dvd by the infamous Caron of Core DVD fame? I have that one and it exhausted me, and that was without even keeping up with her. I think I hurt myself the last time I used it, so it's been a while, but I should try it again.
I do have that dvd but that's not the one I usually do. That one is hard; there are several things - the one arm pushups for example - that I can't do without practicing.
The one that gets me sweaty though is one by Rodney Yee (Power Yoga - Total Body Workout). It moves slowly but just keeps going - no breaks at all.
I haven't done more than the first half for a while though - the last time I tried to do some of the backbends and poses that require bending your knees past 90, I pulled something in one of my thighs so I've been reluctant to try again. One of these days...
I don't remember one arm pushups, so I may have the other dvd - there are two "volumes." There's no way I could do a one arm pushup anyway! :-)
The Rodney Yee one sounds deceptively hard. There are things I can't do, or can't do without hurting something, so I just substitute freely and don't let it worry me. I do that in class too and I still get a good session, so it's OK. The newest teacher seems very aware that people try to push past limitations when they shouldn't, because she always gives a few possible substitutions for each pose, which is nice.
Now that I think harder, it was "pushups" with one leg in the air not one arm. And not "pushups" but rather go from down-dog to chaturanga to up-dog back to down-dog all with the one leg in the air. By the time I would get to those my arms were shot and I could only do one or two before collapsing.
Hmmm...that sounds familiar, except for the one leg in the air part. Probably ignored that! I'll have to get that out and refresh my memory. I've been meaning to give it a try again anyway.