Sure, some events in the OT probably happened, but if I go by how much I exaggerate sometimes when telling a story, the Great Flood probably was just a dam that broke it's banks or something. Also doesn't help that every major culture that's old enough has a Great Flood in their history somewhere. It's almost like it's a TV trope of the ancient times.
Nothing in the OT can be ascribed to christianity alone. Things were much more... "fluid" then, and the concept of organised religion did not exist. Things happened, and people blamed the gods/demons/monsters/spirits/forefathers. Only after the fact were these stories appropriated by organised religion for whatever purpose or reason.
Folks living 100s of years: maybe they lived 100s of full moons, or seasons? Maybe the number was rounded up with every retelling to make the subject of the story appear more heroic? There is just no way that we can ever filter out the fact from the fiction in these old books I think.