To AV1611VET:
When I was taught about the 120 years thing, I was told that it was an explanation for the decrease in lifespan of the people.
Genesis 6:1-8
The flood and the tower of Babel
When mankind began to increase and to spread all over the earth and daughters were born to them, the sons of the gods saw that the daughters of men were beautiful; so they took for themselves such women as they chose. But the Lord said, 'My life-giving spirit shall not remain in man forever; he for his part is mortal flesh; he shall live a hundred and twenty years.'
In those days when the sons of the gods had intercourse with the daughters of men and got children by them, the Nephilim were on the earth. They were the heroes of old, men of renown.
When the Lord saw that man had done much evil on earth and that his thoughts and inclinations were always evil, he was sorry that he had made man on earth, and he was grieved at heart. He said, 'This race of men whom I have created, I will wipe them off the face of the earth -- man and beast, reptiles and birds. I am sorry that I ever made them.' But Noah had won the Lord's favour.
(end scripture)
Note the context in which he declares that man should only live 120 years. It's in the middle of while talking about the sons of the gods or angels or whoever they were having relationships with humans. I was playing a game just recently, and it sort of sticks out to me, because it was talking about a mermaid who had a relationship with a man. But mermaids live much longer than humans (at least in this game) , and he ended up dying much earlier than her. That's notable, don't you think?
The change in lifespans isn't immediate, but as you look on through the list of descendants, it'll show their lifespans slowly going down to around 100 years or so. For example, Abraham lived 175 years, Isaac lived 180 years, Jacob lived for 147 years, Joseph for 110 years, and Moses lived for 120 years.
And another thing to do with context -- it wasn't until verse 5 that it tells about God seeing how evil mankind had become. If the part about the 120 years was really about the amount of time before the flood, then it would have probably been after mentioning this fact.
In my opinion, the Bible doesn't say how long it took Noah to build the ark, or whether Noah tried to save the other people. Unless you decide to read between the lines.
Edit: I re-read the verses you provided from the new testament about Noah preaching to the men. I couldn't find them earlier while I was making my post. I agree that they do say he condemned the others before the flood. I'm guessing this idea probably found its way into the NT from a rabbinic tradition.
As I've said before, I generally accept the historical-critical method of interpreting the Bible, so I don't think that there were people who actually lived that long. But I think what I wrote above still applies to your interpretation.
To TheOutsider:
Look up the Sumerian's list of kings. It's likely that they got the long ages from them. The Sumerians/Babylonians also have a story very similar to that of Noah and the ark. And there's also the mythological tablet of destinies, which grants the person who owns it the power to do anything by simply speaking it. I suspect it is probably the inspiration behind the part in the Priestly creation story where it says God spoke and it was so.
