Ah, good point! Not only did they have to walk & swim all the way to the other continents (eating the bloated carcasses of the drowned animals along the way, IIRC), not one of them ate a bad meal or dropped dead from exhaustion. And needless to say, they all had to get to their far continents within one generation. So, for example, all the small Australian marsupials had to have reached Australia within a couple years.
Now, a creationist could counter that we're only talking two individuals from each kind, plus their children & maybe grandchildren, making the trek across the Earth, so we shouldn't expect to find the fossils of those who died along the way. Problem is, we're talking representatives of 8,000 kinds making the trek. So it would still be hundreds of thousands of individuals in all. You'd think there'd be some evidence of a mass radiation outwards from Mt. Ararat at a specific point in time.