Scalp,Khanbulat & Mirage are the critical lands. All other lands can be done at leisure, they do not affect the Klondike Story.
Secret Vale: special land - very rich in resources - once you make your first trip here you have 7 days only to complete it!! The timer begins when you make your FIRST TRIP to this land. Read the guide first Horton: is no longer in Khan Mires, it will appear in Dragon Wing when you make the map at Mirage (or you have opened this area already)
Special timed land. The timer begins only when you first visit the land.