Have A Mountain To Climb Meaning

(idiomatic) To be faced with a difficult task or challenge.

Example: 2011 October 1, Phil Dawkes, “Sunderland 2 - 2 West Brom”[1], BBC Sport:
  The Black Cats had a mountain to climb after James Morrison's header and Shane Long's neat side-foot finish gave Albion a 2-0 lead five minutes in.