Abraham leads an interesting life. For most of the next ten years he travels to Egypt during a famine, gets himself into some trouble over his wife, and leaves Egypt. Now Abraham is a wealthy man. His nephew is not exactly poor either. They get into a little squabble over the best grazing land. Abraham, who wants to keep the peace, tells Lot to pick where he wants to stay and he'll go the other way. Lot's not good at making choices, obviously, and "...pitches his tent toward Sodom." (Genesis 13:12) God directs Abraham to move on further down the road because the men of Sodom were so wicked. He moves on to the plains of Mamre near ancient Hebron.
By now Abraham is eighty-five. He's starting to worry because the only one to inherit is his steward. (Genesis 15:2) God assures him that this will not be his heir. God again promises him an heir. Sarah is seventy-five, knows the story, and is anxious that Abraham have an heir. (There's no way, or so Sarah thinks, that she will ever bare a child especially at this age!) She convinces Abraham to have a child with her handmaiden, Hagar. So the child of nature, Ishmael, is born. A child of nature because this is not the result of God's promise to Abraham. It's a result of Sarah trying to hurry along the process. As if God needs help.
Things rock along and Ishmael is a teenager. Abraham is almost one hundred and Sarah is ninety years old. God again tells him that he's going to bless Sarah and give her a son. It seems really far-fetched to Abraham and he lets out a laugh. Now, notice this. Abraham laughed first. (Genesis 17:17) Then Sarah hears it again and this time, she laughs. She is way past child-bearing age. Only a miracle could make such a thing happen.
Thus, the child of promise, Isaac, was born to a one-hundred year old man and a ninety year old woman who is well past child-bearing age. Is any thing too hard for the Lord? (Genesis 18:14)