Running at Altitude

I am going to be in Colorado next week (south of Denver) and plan to run a longggg run with some friends. The altitude reads 8,000+ feet. I have literally no experience running at elevation. Am I crazy to think I will be fine to run distance with no acclimation? How long does it take to get acclimated? And will I be fine just going out and sending it? or will I need to bake in some time to get used to the altitude?

THANKS!

Tyler