Wait And Wait Again

Now the Lord had said unto Abram, Get thee out of thy country, and from thy kindred, and from thy father's house, unto a land that I will shew thee: And I will make of thee a great nation, and I will bless thee, and make thy name great; and thou shalt be a blessing: And I will bless them that bless thee, and curse him that curseth thee: and in thee shall all families of the earth be blessed. So Abram departed, as the Lord had spoken unto him; and Lot went with him: and Abram was seventy and five years old when he departed out of Haran. (Genesis 12:1-4 KJV)

When Abraham was 75 years old, God told him to leave his country without telling him where to go and said that he will be a great nation when he had no children.

And the Lord said unto Abram, after that Lot was separated from him, Lift up now thine eyes, and look from the place where thou art northward, and southward, and eastward, and westward: For all the land which thou seest, to thee will I give it, and to thy seed for ever. And I will make thy seed as the dust of the earth: so that if a man can number the dust of the earth, then shall thy seed also be numbered. Arise, walk through the land in the length of it and in the breadth of it; for I will give it unto thee. (Genesis 13:14-17 KJV)

Again God showed Abraham the land and promised Abraham that his descendants will possess the land, but Abraham had no children.

Now the word of the LORD came to him: "This one will not be your heir; instead, one who comes from your own body will be your heir." (Genesis 15:4 HCSB)
On that day the LORD made a covenant with Abram, saying, "To your descendants I have given this land, From the river of Egypt as far as the great river, the river Euphrates: (Genesis 15:18 NASB)

God narrowed down the conditions and state that the heir will be Abraham's physical descendant, not an adopted heir. The boundary of the promised land is also clarified.

So Sarai said to Abram, "The LORD has prevented me from having children. Go and sleep with my servant. Perhaps I can have children through her." And Abram agreed with Sarai's proposal. (Genesis 16:2 NLT)
Abram was eighty-six years old when Hagar bore Ishmael to Abram. (Genesis 16:16 ESV)

After a long wait, Sarah decided to take things into her own hand. She advised Abraham to get an heir through her maid Hagar. After all, God promised that the heir must come from Abraham, but nothing is said about the mother.

But my covenant will I establish with Isaac, which Sarah shall bear unto thee at this set time in the next year. (Genesis 17:21 KJV)

When Abraham was 99 years old, God gave him the name Abraham and his wife was renamed Sarah and the promised heir named Isaac made it clear that the parents of the promised heir must be the Abraham and Sarah. God also set the timing. It will be fulfilled in a year. Abraham and all the males (born or bought) of his household was circumcised on that day.

And Abraham was an hundred years old, when his son Isaac was born unto him. (Genesis 21:5 KJV)

God took 25 years to fulfill His promise, from His first promise to Abraham when he was still Abram and 75 years old about being father of a great nation to the birth of Isaac when Abram became Abraham at 100 years old. God worked very slowly and tested the faith and patience of Abraham and Sarah.

So the field and the cave were transferred from the Hittites to Abraham for use as a permanent burial place. (Genesis 23:20 NLT)

Finally, Abraham got to own a piece of the promised land, and he had to buy it from the Hittites.

When they therefore were come together, they asked of him, saying, Lord, wilt thou at this time restore again the kingdom to Israel? (Acts 1:6 KJV)

These all died in faith without having received the promises, but they saw them from a distance, greeted them, and confessed that they were foreigners and temporary residents on the earth. (Hebrews 11:13 HCSB)