I went up to Denver this week for DrupalCamp Colorado, and of course we had to go up a day early to do some actual mountain biking. We call the offroad riding we do here in Dallas "mountain biking", but with maybe 30-40 feet of elevation change in a typical trail, it isn't really.
But the Red Rocks trail in Golden, CO really is. We were invited on the ride by Randy Fay, a real endurance rider and met at the part bright and early at 7 am. After about three miles of up and down short and rocky stuff, we hit the big 400 foot descent. Going down I can handle, wasn't too sure about going back up.
We turned around at the famous Red Rocks ampitheater, which was just spectaular- had only seen it on television before. The climb back was brutal, and may have involved some walking up particular steep and nasty parts, but was totally worth it. One of the best rides ever.