In reply to gavmac:
I tried 'running' up Bear Peak (I think) near Boulder last year. I went up and down the same way, but I think there might be variations of loops one could engineer. It's quite far removed from what passes as a trail-run in my world, however, and the relative thinness of the air was strikingly apparent.
It's a bit to the south of Boulder near the Table Mesa area, which might be helpful for access from Denver.