The general rule:
Two weeks out (basically now): Cut your training volume in half. Do not cut the intensity, just the time/distance.
One Week Out (Starting Saturday): Cut the volume in half again. (That 20+ mile becomes 5)
The basic idea is that at this point the odds of improving your performance are low but the risk of getting injured with insufficient time to recover is high. Or put another way: The hay is in the barn.
Experienced folks will have a better idea of how much and when to start taper. But for a first timer, keep it simple.
As for altitude, I'd spend as much time up there as possible. I've known of folks who would go up to the summit, go down 2 and then back up (vs a full 3-2-1), and then break out the lawn chairs and read a book. :-)