Alone.  In the comfort and privacy of my own home.  After I've had 1.5 glasses of Pinot Noir.  

This is supposed to work your core and legs, and I'm guessing these people make it look a LOT easier than it really is.  But it's more fun than running 3 miles and getting caught singing along to your iPod by your neighbor you didn't know was there.  

I mean, that never happened...