I believe the intention was that they start in the garden. As there were more people born, the task of expanding the garden across the Earth would have become easier.
Of course that could be possible, but it requires quite a suspension of "timeline" to be ok with it... although this is what I was taught from kindergarten on up through my whole Christian private school education.
This is why I started with my hypothesis in the first place.... the "timing" didn't work, when you say that the creation of Adam and Eve is just the "detailed" description of the 6th day....
When you entertain the idea that God created "mankind" on the 6th day, and told them to go forth into all the earth and be fruitful and multiply, and to subdue all the animals and fish of the sea..... and then he Created Adam (not mankind, but son of God) specifically to live in the garden, then a whole lot of other questions have viable answers...
Why did Cain worry that "whoever finds him will kill him" if he and Abel were the first two children born to Adam and Eve after leaving the garden. Who would have he been worried about? How many generations of kids would Adam and Eve had to have produced to ensure that there were "people" out there that might kill him?
Also, in the same vein, who did Cain take as a wife? If we assume Adam's family all lived in the Eden area (not the garden), and Cain was driven out into Nod, who lived there?
It would also help explain the odd description of the sons of God taking the daughters of man as wives....
There are many other questions that could be asked, and my hypothesis would answer most of them....
I have no problem with blind faith.... faith becomes the substance of things not seen.... but we also should be willing to think about things with an open mind.. My hypothesis would not change the scripture at all, it would simply change the understanding of it...