You could easily do any of those trips! I'm a slow girl rider your age. My husband and I trained for this summer's TransAm in totally flat Yuma, AZ (laser planed for agricultural irrigation). I ride a mountain bike with the whole anti-flat 2" slimed, linered tire setup and go a whopping 8-12 mph most of the time. 3 mph up mountain passes, 14-15 with a tailwind :-)
Our training ride was 50 miles, no hills. Don has COPD so we took a weekend trip up to 6,000 feet to see what would happen & he was fine (albeit on a Trek electric-assist bike, FX+). I've also done AC's Northern Tier and its discontinued North Star (Missoula to Anchorage, AK, half dirt roads) with 50 mile training rides in Mesa, AZ with only one hill. Not ever having ridden with a club, I was totally unfamiliar with the concept of spinning as I hit my first ever mountains, the Cascades, on my first ever tour, the Northern Tier. The group leader took one look, told me to get down in my lowest gear (luckily a mountain bike has really low gears) and spin! I did and went right over the Cascades and Rockies. I don't think I've ever stood up climbing hills and have never (yet) walked one. Just make sure you get a bike with low gears!!
My understanding of high altitude disease is that it's random - fit people can suffer just like the unfit. I was afraid COPD husband would have trouble on Hoosier Pass (11,500') but it turned out to be no problem. Cycling apparently isn't as oxygen demanding as hiking because Don would have to stop and catch his breath once or twice on the ~ 500' walk uphill from the Keystone convention center to our hotel room. It helped that we had gradually acclimated by cycling over progressively higher passes coming from the west coast. Since you will not have this luxury, here are 3 ideas. 1) several people in the ski town of Keystone, CO told us it helps to SLEEP at high elevation before exercising there, so maybe you could come a few days early. 2) we bought packets of a powdered Gatorade - type product that is supposed to prevent altitude sickness called Acli-Mate at the Grant Lodge store in Yellowstone 3) avoid caffeine on altitude days.