Prior to my step, I had only taken one other 8-hour exam, and that was the obnoxious MCAT. It suffices to say that I really, really don't like 8-hour exams. I hate them so much I won't even spend 8 hours taking a mock exam. So, I just worked question blocks from different review sources as I felt like it, all the while thinking about how if I really was going to spend 8 hours taking the actual step (which I didn't plan on), I only wanted to go through that ONCE.
Looking back, I'm glad I didn't work any full-length mock exam. It wouldn't have helped me, and I really thought working questions at my leisure was much more useful.