God would not leave Adam and Eve destitute. The promise God made in this chapter is called the proto-evangel, the first prophecy of the Messiah. Understanding Who the Messiah would be, and what He would do, was going to slowly unfold throughout the Old Testament. Then, throughout the New Testament, Who Jesus is and what He has done is revealed.
Imagine the moment they entered the hushed glade where Life and Knowledge stood, in their quiet power. The Tree of Life, he told her. We may eat of all the trees in the Garden, including this tree. But already she was looking at the other, beguiling, Tree, spellbound by its exotic loveliness, its alluring fragrance redolent with rare spices, sharp and tangy, the perfume of hidden mysteries.