I don't even try to understand the genealogy of Old testament bible stories. It's a great way to get a headache, even before applying logic to it. (I don't mean this as a religious slam, I just mean that post Cain and Able you need to go at it with post-it notes just to remember who's who and what's what. Then again, it's been a pretty dang long time since I read that part of the bible.) The Lineage stuff gets kind of confusing, but yah, lots and lots of inbreeding.
Odd speculation: If God gave people those crazy long life spans, maybe he could add some crazy mutations or something. Lack of genetic diversity is a huge problem with endangered animals, and makes them more susceptible to environmental changes and diseases, hence increases the rate at which they become extinct.