Originally Posted by thatollie
Just write the dang thing.
Kind of agree with this sentiment. Write it and see where it takes you.
On the destruction issue, I have to say nuclear war, on any significant size would be so devastating, for such a long time, that you're looking at massive contamination for hundreds of thousands of years. You could always consider the old asteroid strike scenario (medium size). Equal devastation, but less long term contamination.
At the end of the day, though, if so much has been lost, the destruction may all be mythologised and its true causes unknown to your characters. Like the idea of your protagonists being myth collectors, by the way.
As to length of time for history to become legend and myth, look at our own past. If electronic records were redundant and libraries destroyed (a terrible thought for all of us, I'm sure), I suspect it would only take a couple of thousand years. At the same time there might be some small groups, such as your protagonists guarding the knowledge and books that remain.
So, just start writing it. See where it goes and if you need to, change stuff in the re-writes. Good luck.