I'll quote Genesis through and through to demonstrate that I'm not spinning the story, but rather simply describing what it says.
Another additional concept to understand is that when God created the heavens and the Earth Genesis 1:1, God then subsequently brings order to that which was formless and void Genesis 1:2. God separates light and darkness Genesis 1:4, He separates water from water Genesis 1:6, he gathers the waters below to form dry land Genesis 1:9. God establishes an order in an otherwise chaotic world. (All of the above)
So there is earth (or otherwise translated as a region of dry land in which people lived) then on earth there is Eden, then within Eden there is the garden of Eden, then within the Garden there is a the tree of life and the tree of knowledge of good and evil (Genesis 2:8-9). And God creates a dome/firmament in the beginning over the dry land, and this dome/firmament holds back chaos from above, and the land is a barrier to chaos from below, chaos being in the form of the flood waters of Noah. (Genesis 1:7-9). And eventually Adam is thrown out of the garden, essentially into the chaos (Genesis 3:23-24). The flood story also plays on this same concept where chaos is basically reintroduced to God's orderly creation as water passes through the windows of the firmament and through the earthly barriers which God had created to hold back the chaos (Genesis 7:11).
And so when Adam and Eve sin and are kicked out of the garden (Genesis 3:23-24), they're then removed from the orderly, into the world of desolation, thorns, thistles and corruption.
And thorns and thistles shall sprout for you, and you
shall eat the plants of the field. By the sweat of your brow you shall eat bread, until your return to the ground.
For from it you were taken;
for you are dust, and to dust you shall return.”
Genesis 3:18-19
In this view, it's not so much that death didn't exist prior to the fall, rather the tree of life inside this garden and under this firmament, was a place of sustained order. Hence why God blocks Adam from returning to the tree of life with use of a flaming sword, lest he eat of the tree and live forever (Genesis 3:22-24). He's been kicked out of God's firmament, and left out, separated from God in this chaotic formless and void place. Kicked out of God's house essentially. And Jesus will one day return to bring us back into His house to sit at his table as his heirs.
Theologically it just makes too much sense that death would exist prior to the fall. In some manner. And theologians throughout history have supported this position. It's not that bad things, void and desert never existed before the fall. Rather God simply held them back as He did with the waters above the firmament.
God intended for people to be fruitful and to multiply and fill the land. To make earth as it is in heaven. So God is commissioning us to bring order to the void (Genesis 1:28). Though something went disastrously wrong when Adam allowed that disorder to take hold of his heart while inside the garden (Genesis 3:6). So God kicked him out of the garden (Genesis 3:14).
Then of course Adam and Eve have children and Cain is further exiled from Eden, hence why Cain becomes fearful for his life. Because he knows that death and chaos awaits him (Genesis 4:12-14). Who was Cain afraid of that would kill him if Adam and Eve and their direct children were the only people on earth? The answer is that Cain feared human beings beyond Eden in the dark and desolate void. He's saying that it's dangerous out there, please don't exile me from Eden.
Genesis 2:8 and God planted the garden within Eden. Not out in the void and disorderly, but within the space in which He established order, inside Eden. And there He put the Man whom He had formed. So Adam was made outside of Eden, then put into Eden (Genesis 2:8), hence why God says that to dust Adam shall return (to dust outside of the sustained Eden and into the spiritless, broken void). So God brought Adam out of the chaos and death and destruction and brought Him into the Garden by the tree of life where He might be able to sustain his mortal body. Adam sinned, then got kicked out back into the land of thorns and thistles.
Then when God kicks Adam out, God says that to dust Adam shall return. No longer protected with the tree of life, but pushed out into the realm of unprotected disorder, where death exists and had always existed from the beginning. Genesis 3:19.
And remember, the tree of life according to scripture grants eternal life. If death didn't exist, it's hard to imagine why God would even have such a tree. So when God stops Adam from eating of the tree of life, it means certain death for Adam, just as God promised. Not because God made Adam mortal, but because Adam was always mortal, and was simply sustained by the tree. Adam put his trust in the tree of humanly knowledge of good and evil rather than putting his trust in God's tree of eternal life. So God kicked him out of the garden, put a flaming sword up to guard the tree, and Adam was then destined to certainly die, just as God told him he would if he ate of the wrong tree.
And this is what Genesis says. It's a story about putting trust in human knowledge over God's wisdom, a fall In brokenness, then Jesus returns to redeem us and to bring us back to a new garden of Eden.
The scripture affirms it, it's theologically sound, and the science affirms it too. All indications suggest that physical death existed before the fall. Saint Thomas Aquinas, an army of theologians and scientists affirm it. The scripture appears to say it, and that's good enough for me. At the very least I've taken science, theology and scripture and they all align under this worldview. So, can't blame a guy for "running with it".
The extended version: