I will be coming from CT at sea level . I am a bit anxious about hiking in the high altitude.
I'm not on this trip (although the itinerary looks wonderful!) but I would be a bit concerned as well. I live at 2800' and sometimes have some mild symptoms when I go to Yellowstone at 6,000'.
I'd probably want to arrive in Denver a couple of days ahead of time and get to Estes Park at least the day before to give you a head start on adjusting to elevation.
I am from CT and when in Breckenridge CO (9600 feet) learned to drink a ton more water than I needed, I had no symptoms after super hydrating.