Altitude sickness

I flew out to Denver yesterday and then drove up the mountains to Breckenridge, which sits at over 9,600 feet above sea level.  I have never been this high other than traveling inside a pressurize airplane cabin and I didn’t realize how much the altitude would effect me.  All day yesterday I had a killer headache and was short of breath doing the most basic things.  I can’t imagine someone flying in here from near sea level and then trying to ski.
Today is a bit better, but I still get a little winded climbing up stairs.  Fortunately it seems that the effects only last about 2-3 days, so about the time I’m heading back tomorrow, I should feel much better.