In The Hobbit, what are the "call to adventure" and "meeting the mentor" segments of the Hero's Journey?

In the Hero’s Journey, the hero is an ordinary person who
embarks on a journey of self-discovery to become a hero.  In The
Hobbit, Bilbo is a happy-go-lucky and somewhat hapless Hobbit who ends up
going on an adventure because his friend Gandalf the wizard needs a
burglar.

The hero’s journey always begins with a call to
adventure.  The hero can either reject or accept the call.  More often than not, the
hero (who is not a hero yet) rejects the call.  Bilbo rejects Gandalf’s request, and
Gandalf leaves a note on Biblo’s door for the dwarves.  Bilbo is much more interested in
eating and drinking than in adventures, but Gandalf does not allow him to reject the
call to adventure.

Gandalf is Biblo’s mentor for several
reasons.  First of all, he is the one that presents the call to adventure to Bilbo and
does not allow him to reject it.  He also starts Bilbo on his journey, and throughout
the journey he provides him assistance.  In order for the hero to grow he needs to
separated from, and then reunited with, his mentor.  Gandalf leaves Bilbo but does
return later.