Well, EBarnacle, when you have a mystically significant number like 40, the meaning is more significant than the actual passage of time. 40 indicates a time of retreat and reflection. But note that in the story, the people spent about a year traveling from Egypt to Palestine - and 39 years settled at the base of Mount Sinai (wherever that was). Guesstimates are that the Exodus took place about 1200 BCE. The stories were passed alone orally for three or four hundred years before they first appeared in written form, and then the Pentateuch went through maybe another three hundred years of editing and rewriting before it got to more-or-less its current form. Some say the current form of the Pentateuch dates back to sometime after the return from Babylon, which was in 535 BCE.
Some say that Exodus is as much a retelling of the exile in Babylon, as it is an account of the exile in Egypt. At the very least, I think it can be said that the Exodus event, as written, is seen through the eyes of those who have more recently experienced the trauma of the exile in Babylon.
This all makes the Bible a fascinating document to study. We tend to forget, however, that it is an ancient document produced in an ancient context. You just can't take it off the shelf and let every preacher with a dimestore diploma use it as a book of instructions for daily living. There is much in the Bible that is clear and quite inspiring - but there's a lot that demands a great deal of scholarship to approach any level of understanding.
So when people tell me that the Bible prohibits homosexuality and a bunch of other stuff they don't like, I politely change to topic of discussion.

-Joe-