I went to a total body conditioning class and had to leave half way through. It wasn't that it was too hard, in fact it felt great. But 15 minutes in I thought I was going to loose my breakfast. It was awful, but I had to listen to my body. I could choose to either sit down, stop moving or throw up or worse blackout. I figured the option of bowing out in front of women twice my age was better then passing out in front of women twice my age.
So here is a tip: If you haven't worked out in a week or more take it slow or else you might just embarrase yourself.