Adam lived for 800 years?! Good grief, the Bible's authors certainly had vivid imaginations.
In the OT (specifically the Torah), extreme longevity was used as a way to communicate the idea that people were very blessed... it quickly disappeared after those original writings had been taken down, and attitudes toward Earthly blessing changed. The point they are getting accross is just that this was someone who had God's favor (yes, even in Adam's case).
Interestingly, Tolkien used the same literary tool for the 'good' humans of Middle Earth. Aragorn, for example, lived to the age of 210.
