Drill stars are awarded throughout the session depending on how you do for each drill. The workout stars are awarded based on your cumulative fitness points (FP) for the session. To acquire this achievement, you simply need to have 50 workout stars (10 perfect 5 star sessions or any combination... i.e. Seven 5 star sessions, three 4 star sessions & one 3 star session).
I can confirm that you can replay a session you already completed and those will count as brand new stars.