Genesis 5:3-4 And Adam lived an hundred and thirty years, and begat a son in his own likeness, after his image; and called his name Seth: And the days of Adam after he had begotten Seth were eight hundred years: and he begat sons and daughters:
Lifespans were long. Our ancestors genetic material had less defects than they do today. And close relatives could marry without creating 'impaired' (e.g., Carter-esque) offspring. So we are the ancestors of Adam and Eve and the product of the marriages between their off-spring. But you don't need to go as far back as Adam - you just need to go back to the 8 on the Ark: Noah and Mrs. Noah, Shem and Mrs. Shem, Japeth and Mrs. Japeth, and Ham and Mrs. Ham....
Why does this matter? Care less about your family tree, and more about whether you are a new creation in Christ.
2 Peter 1:10-11 Wherefore the rather, brethren, give diligence to make your calling and election sure: for if ye do these things, ye shall never fall: For so an entrance shall be ministered unto you abundantly into the everlasting kingdom of our Lord and Saviour Jesus Christ.
Why do global patterns of mtDNA distribution not show this?
There is a skeleton from southern Alaska, dated to just over 10,000 years old. The mtDNA pattern is identical to extant local tribes, not a Middle Eastern pattern.
This suggests no break at the date ascribed to the global flood (ca. 2350 BC) and no replacement of earlier mtDNA patterns with the mtDNA of survivors of a global flood. (There are other examples of mtDNA continuity from the western US as well.)