And Hashem said to Avram, “Go (to) (for) yourself, from your land, from your birth place, from your father’s house, to the land that I will show you. (Breishit 12:1) 

Go,yourself,' A hint to him, 'When you will be 100 years old (30+20+30+20), then I will make you a great nation.' For then Isaac will be born. Baal HaTurim 

G'd did not tell Abraham if he was meant to set out on his journey immediately or if he should wait till He would specify the exact location He wanted Abraham to move to. This ambiguity was part of the test to which G'd subjected Abraham. G'd also hinted
that He would show Abraham the whole of the land of Israel by broadening his field of vision, etc, as we know from 13,14: 'lift your eyes from where you are and look northward, southward,
eastward, and westward. Or_HaChaim