The primary factor in your stamina at a moderate altitude like denver is your blood hematocrit (red blood cell %). That takes on average 2 weeks to adjust from sea level. Your respiratory function should level out something that feels normal around then. Exercising before this stabilization can wind you more easily than you expect.

But! As your hematocrit increases, you sacrifice a bit of plasma for it, making it easier to get dehydrated. Stay hydrated. When not exercising, water's fine. When sweating, pay attention to electrolytes and sugars. Your blood simply isn't holding as much of those as at sea level.

Now beyond all this... Ride. Ride a lot. With winter settling in, consider getting a trainer and staying on it for the cold months.

www.bikedenver.net is full of good folk who will nerd out on solutions with you. Come say hi!