The broadcasts of the Denver Broncos home games always include a comment about the altitude. I can imagine an August game here would test any athlete. Many athletes train in Colorado.
By the way, the Denver area has gorgeous trails. The Denver downtown area has been transformed in recent years and the Platte River areas is something to behold. Boulder has wonderful trails, too.
As Kristal says, Denver is very athletic.
PML of Longmont, CO
Here are some websites that might help you find the best trails:
Prudential Real Estate of the Rockies