He's quite prepared in a Hero's Journey sense, though. In Harry's own mind, he has lost his mentor. Thus, he is now free to be a mentor. And what better way to grow, as a Hero and über-rationalist, than to teach others to do what you do?

Of course, Harry would say that he's already doing that with Draco—but in the same way that he usually holds back his near-mode instrumental-rationalist dark side, he's holding back the kind of insights that Draco would need to think the way Harry thinks; Harry is training Draco to be a scientist, but not an instrumental rationalist, and therefore, in the context of the story, not a Hero. (To put it another way: Draco will never one-box. He's a virtue-ethicist who is more concerned with "rationality" as just another virtue than with winning per se.)

Mentoring Hermione would be an entirely different matter: he would basically have to instill a dark side into her. Quirrel taught Harry how to lose—Harry would have to teach Hermione how to win.

If Eliezer has planned MoR as a five-act heroic fantasy, it will probably go like this; usually, in a five-act form, acts 4 and 5 mirror the character developments of the Hero in 2 and 3 in another character, for the purposes of re-examining the (developed, and now mostly stagnant) Hero's growth and revealing by juxtaposition what using that particular character as Hero brought to the journey.

It seems more likely to be a three-act form at this point, though, with Azkaban as the central, act 2 ordeal. That's not to say the story is more than half-over already, though; Harry has just found his motivation for acting instead of reacting (to change the magical world such that Azkaban is no longer a part of it.)