Hmm. Guess I'll be the first Christian to tackle this one. Note I'm by no means an expert, but a hobbiest. This is my second post on the website too! Woo.
Well, obviously around this time these people lived a long time (Methusala got to around 900 something, right?) Can't exactly remember if it was before or after the flood that the book says God shortened man's lifespan to 120 years, but that solves your time issue either way.
Secondly, its safe to assume that the genetic diversity was still rather prominant in the Ark's occupants (unless you believe there was an asian on the Ark, or that Ham was actually black, or something of the like). Adam and Eve's population of the planet with so many different races is due to the fact that these first animals had a gene pool that we couldn't begin to fathom, and that all the races of the world may even have been combined into this proto-humanity race of perfection. In either case, 8 occupants had enough time, and enough genes, to produce many children, who could then even breed with siblings (once again, big gene pool here), but didn't necesarily have to with the others. Even now, it takes 10 generations of inbreeding to result in some sort of deficiency in the gene pool, so it's obvious that these remaining occupants would have no problem.
Of course I'm not going to get into the geneology of mankind since Noah or something, but there's a relatively adequate explaination from a mere hobbiest.
Happy ... umm ... evolutionisting!