Strangely enough, your level of physical conditioning has no bearing on whether or not you will get altitude sickness, so even if you are a svelte marathon
runner, you may still get symptoms. In general, the altitudes in colorado (around 14000 feet) are not enough to make you severely ill, but you may still get headaches, shortness of breath and fatigue.
First, acclimatize slowly, the longer it takes you to get to altitude, and the longer you can stay at altitudes near what you will be hiking
at, the better. A reasonable regimen might be, stay at 7-8000 feet for 2 nights, 10000 feet for 2 nights and 12000 feet for 2 nights. Of course, that would require significant planning and put unreasonable restraints on your trip, but the longer you can be at altitude before exertion the better. Hydration
is very important, so drink plenty of water. Avoid liquids that will tend to dehydrate such as caffeine
There are prescription medications that will help with symptoms too, such as acetazolamide
) but these will require a prescription.