Expect to drop down one gear on your cassette from what you'd normally use for a specific type of terrain (I'm assuming you'll be riding on paved roadway?).

FWIW: Track and field has standards for altitude, where they asterick a time/distance/height set above a certain altitude. I haven't looked it up lately but it used to be 1500m/4900 ft for sprint and field events, and 1000m/3000ft for longer running events.