Some wrong ideas in the previous posts, re tree of life, but no worries hopefully.
How long it took them to "actually die" is a very human , but unspiritual , concept.
Even in human terms, like if a mob boss in New York City said of someone "You're a dead man".... well, they would know that they were dead then, even if it was anywhere from an hour, to 40 years later they were executed...
or if poisoned, in some particular way (quite common in the usa, btw), someone could 'know' they would be dead from it, though perhaps after decades, not right away obvious, ......
More to the point actually - When God says something, it is truth. It is unchangeable. When God Says we were crucified with Christ before the creation of the world, it is true already, without explanation, and without contradicting anything. i.e. it does not mean we had any existence before we were conceived (that's a known heresy). It plainly means that God's Plan of Salvation, including our existence once far in the future (as humans consider time), was 'guaranteed' - no possible chance God's Word ever not being accomplished as He Planned, Purposed, and Spoke.
Even all the universe and all on and in the world - was spoken by God into existence.... with no variableness - His Word is So Totally Perfect and Complete, without error, without any iniquity, without any failure at all to be as He Says.
What God spoke centuries ago, is absolutely certainly truth, centuries later - being accomplished in its time as He Decided. (I think this is specified similar in places in Scripture - someone is born, or dies, or goes into slavery, or rules a country, a people, or prevents extravagant loss from a 7 year famine, exactly as Yahweh Planned and Purposed all along since scheduling all the days , before creating anything at all, perfectly) ....