I would not worry about the altitude. Just stay in shape and keep doing the cardio as others above have said.
A few summers ago we took a vacation to Colorado Springs. I wanted my son to see the Academy (he accepted his appointment for the class of 2021 back in November). Two days after we arrived, we found ourselves at the Cog Railroad that goes up to Pikes Peak. Adjacent to that is a fitness trail called the Manitou Incline. After coming back down from Pikes Peak, my son ran up the Manitou Incline without any altitude issues (the top of the incline is some 8000 feet above sea level) and we had only been in town for 2 days, coming from our city that is only 650 above seal level. Point is if you are in good shape, you will notice the elevation, but it won't be an issue.
Here is a link to the Manitou Incline: http://www.pikes-peak.com/want-climb-manitou-incline/
Took my son 29 minutes (when he was 15 years old). Took me 90 minutes...hardest thing I have ever done aside from a full marathon!