Well, the flood was many years later you know. Tell us where they went then?
True, the Flood would have been many years after the casting out of Eden, a little over 1,000 going by geneology of Seth. Bible says Adam and Eve went eastward out of Eden.
We can in no ways know what landmasses existed in the pre-Flood Age due to the Flood wiping out and rearranging the whole world. Such a cataclysm would fundamentally change the entire face of the earth. Furthermore we know that Noah, Shem, Ham, and Japheth and their wives are the only people to have survived the Flood. We also know Seth died well before the Flood, and thus as he was younger than Cain, if Cain lived out his full lifespan would have died also before the Flood (though it is unclear if Cain lived a full lifespan or if he was murdered given his progeny, Lamech's boast.)