The Four Passes Loop (nicer name: “Ring of Bells”), is considered one of the best long trail runs in North America. Starting at 9,600′ at the postcard-perfect Maroon Lake above Aspen, this loop is almost exactly Marathon distance, and transects the Maroon Bells/Snowmass Wilderness Area while going over four big passes, each over 12,400′. The scenery is amazing. There are no signs of civilization; only a runnable single track trail.
There are two ideal times to do it:
1) Late July, when the entire route is blooming with wildflowers; or,
2) The third week in September, at the height of the changing Aspen colors.