Is teaching a graduate course with 30 students an unreasonable expectation for a PhD student with a TA contract?
It is incredibly common for the number of hours devoted to teaching be woefully inadequate to deliver good instruction except possibly for highly seasoned instructors teaching the exact same course multiple times.

Unfortunately, I don't know any fair solution to this.  You'll probably spend two or three times longer than the expected number of hours in order to do even an adequate job.  From what I observe, people cope either by working far more than they're supposed to, or by delivering not-very-good instruction.  It's a difficult spot to be in.  (Alas, not the only difficult spot in academia.)

The bright side is that if you're really interested in teaching and put in way more time than you're "supposed" to, next time it won't be as bad (you'll only have to put in somewhat more than you're supposed to), and if you do well you will both help your students and be known as someone who is a good teacher (good if you want to do more teaching).