I grew up in PA but moved to CO after getting married. "Flatlanders" (as we like to call them) usually take a day or two to adjust depending on how far into the mountains they go and how much physical activity they're doing. Drink lots of water (it'll help with the headaches). For me personally, it didn't take very long to adjust (no more so than your average person). I even ended up hiking two 14ers (14,000+ ft) while I lived there and felt fine. I actually think the high altitude was better for my lungs, it strengthened them over time.
For a short vacation, just drink water, take it easy the first day or so and you should be fine. Enjoy! Colorado is gorgeous country!
For a short vacation, just drink water, take it easy the first day or so and you should be fine. Enjoy! Colorado is gorgeous country!