Ah you mean exam length, not course length. I don't remember a GCSE exam that went beyond 2 hours. There may likely be some now with the new courses in place though. The shortest you'll get an exam is likely an hour. I haven't seen the mocks that you were given, so it's very difficult to tell you whether they are easier or harder than the real exam or by how much harder.
Exam questions are supposed to be without prior warning, you're not supposed to know what the questions will be, and you're in the same position as everyone else.