The Emerald Lake Trail is a popular hiking trail located in Rocky Mountain National Park in Colorado, USA. The trail is approximately 3.6 miles (5.8 km) one-way and has an elevation gain of about 650 feet (198 meters), with a maximum elevation of 10,110 feet (3,081 meters) at Emerald Lake.

For a trail run under 10 miles that offers a challenge with elevation changes and a mix of easy and hard sections, I would recommend the Mount Sanitas Trail in Boulder, Colorado. This trail is approximately 3.3 miles (5.3 km) round trip with an elevation gain of about 1,343 feet (409 meters), with a maximum elevation of 6,863 feet (2,092 meters) at the summit. It offers a mix of steep and rocky sections interspersed with flatter sections, making it a challenging but runnable trail. However, please keep in mind that the trail can be crowded, especially on weekends.
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