Karen,
Hate to say it, but I can't think of any way to train for elevation without having elevation to train on. It takes a good month to really acclimatize to higher elevations. I would think the best thing you can do is to train as much as possible, get your legs and lungs as strong as you can before you go and then just enjoy once you get there. Sounds like you are already doing that. I also live in the flatlands, no elevation, right next to the Mississippi river. I rode in Colorado last summer. Loved it, but definitely noticed the lack of oxygen! We mostly rode on the road, but did a day of mountain biking at Crested Butte. Awesomely beautiful area, but climbing on the mountain bike gave me a new max heart rate! I had to stop partway up and catch my breath. Once we were up, tho', it was okay. And I love hills! You just go, do what you can do, and enjoy being on a bike. :-)
Oh, and make sure you stay well-hydrated, before and during your trip. It will help prevent altitude sickness.
Have fun!
Time is a companion that goes with us on a journey. It reminds us to cherish each moment, because it will never come again. What we leave behind is not as important as how we have lived." Captain Jean Luc Picard