How long were Adam and Eve in the garden, before Satan showed up? After all, they did have access to the Tree of Life, didn't they?
Personally I don't think they were there for billions of years. I think it is more likely that, just as God created adult human beings, He probably created an "old" earth and universe. He could have done it, couldn't He? Anyhow, I consider myself a scientist (PhD in molecular biology, published research...) yet I believe in a literal interpretation of Genesis.
The world was created five minutes ago, complete with our memories.