Last evening was a 2 1/2 hour hip, Iyengar, workshop with Bodil Mauritzen. We did standing- and «on our backs» asanas. It i important to keep the legs straight, so that you activate the leg in the hipsocket.
Further it is important to access each yogaclass as it is your first time. Don´t compared it to what you have done before or what others can do, just focus on your body.
Yogis believe that emotions are locked in our bodies. So to do only hips or shoulders get you out of balance. It is common for a beginner to do all round classes, then you might focus on stiff areas, and after that you return to an all round classes. The key is balance and stamnia.