Don't give up, it really works. You will see results immediately especially if you are significantly overweight like I am. When it gets hard, she keeps reminding you you won't be able to do any other workout for just 20 minutes and see results. She is totally right.
My advice would be to do another workout for a couple of weeks before starting the Shred lol. It almost killed me (literally, I had an asthma attack for the first time since I was a teenager). I plan to start it again after I've gotten used to being more active (probably around the beginning of March). Other than that, just stick with it. I've seen people get some killer results from it.