This section of the Encampment River Trail follows the river to the northern boundary of the Encampment River Wilderness/Medicine Bow-Routt National Forest. You could keep running, if you like, and complete more than a marathon in round-trip distance before reaching the last trailhead.
With that said, keep an eye out for reports of downed trees, which are common along this trail. It's nice to know whether you're in for a pleasant trail run or a steeplechase.