Friends and I hiked several 14'ers when I was with my dad, when he was stationed at Ft Carson CO (Colorado springs). It was also about this time I took up track in high school. During any kind of strenuous physical activity the altitude will get to you at first, depending on where are coming from. The thing that kinda sucks though, is if your on vacation by the time you become acclimated you will be going home.
Just remember when your sucking air that Mt Everest (8800m) has been climbed many times without supplemental oxygen.