They believed (after some doubts) the promises of God. And when the promises were not (yet) fulfilled, they considered God faithful who had promised (Hebrews 11:11). And when it was his time, God miraculously fulfilled his promises (Hebrews 11:12 and Genesis 15:5; Genesis 22:17). In this way Abraham and Sarah are models of the endurance in faith that will see the fulfillment of the promise.
11 By faith Sarah herself received power to conceive, even when she was past the age, since she considered him faithful who had promised.