This is my second review of this excellent program. I often like to do a review based on my first impression and then later after living with a program for a while to see how I feel over time.
I've done this routine quite a few times since I purchased it and it continues to be a very challenging routine. It focuses a lot on the lower body, in particular the hips, through lunge type movements (crescent lunges, warrior 2, reverse crescent lunge, side stretch) as well as "chair" movements, which are squat movements. Much of this is also tied in by building through Sun Salutation B.
This is definitely not a routine for someone who just wants to "coast" along because your legs will "burn" (at least mine do:)) and you'll truly need to connect with your breath (if you want to benefit in the truest sense of yoga) to maintain some semblence of equanimity in the midst of these movements:)
Of course the pain should only ever be on the discomfort level (and it can be fairly uncomfortable :) ) without ever crossing over into a level of pain that could result in injury. It is important to maintain complete awareness (which is the main point of Yoga anyway).
One thing you can do is to start a little above the discomfort and then ease into the pose more and more as you breathe so that you can open up safely. Yoga is definitely NOT a "no-pain no-gain" practice so leave that sort of "gym mentality" behind if you are new to yoga.
If you are looking for a challenging and yet still "real" yoga practice with an inspiring and knowledgeable teacher I highly recommend this program as well as the other 3 in the "Sacred Yoga" series.
I would suggest buying the 4 program set and you'll have enough variety to work with for a long time (variety in terms of challenges, keeping yourself fresh and excited about your practice so that it doesn't become predictable and variety in terms of being able to respect and work with your energy level).