I'm holding a private ultra altitude training camp in 2 weeks. I'm pretty happy with the setup: a bike path around Dillon reservoir, 18 miles around.
I know some trail running aficionados would think this stupid, but I can't run trails. So, this beautiful bike path at 9,000+ feet is where I'll be. Works for me. All photos copied from the internet.
Besides the running, I'll also hike on the Continental Divide trail leaving out of Silverthorne.