A few days ago I went to a class that didn't seem very hard at the time. The pace was leisurely, frequent rest was encouraged, vinyasas were optional. So why did I wake up the next morning to sore quads? Well, the nice, slow pace had us lingering in poses like warrior II so long that I went deeper and stayed longer than I would have in a faster class. It's a good reason to mix up your classes. If you always do a fast practice, you may be missing out on opportunities to discover something new about your body.