This question assumes that the world before the flood was like the world is today with animals specialized for certain areas. Today the world is 70% water and the oceans separate the continents. Also, some animals only live in a few selected locations. The Bible teaches that before the flood the water was gathered into one place (Genesis 1:9). There was probably one ocean and much more landmass. Also, if the climate was more temperate, animals could live in all types of places, which means Noah did not have to go gather animals from all over the world. In fact, the Bible says that the animals came to Noah (Genesis 6:20).