From the intersection with State Road 38 in Red River, drive 6 miles up 578 to the junction with Middle Fork Road on the right and 58A to the left across a bridge. Either park here and run, or drive 1.1 miles up Middle Fork road to a parking area.
From the parking area begin running up the Middle Fork Trail/Forest Road 487. You'll pass a gated property on the right - stay on the trail to the left and cross the Red River on the bridge or run across the creek during low flows. For the next mile run up this trail (it's an old, rocky jeep road) as it climbs up the mountainside on the east side of the creek. When it crosses the water to the right, there's a sign for Lost Lake on the left; this is where Trail #91 begins. Follow this trail up several well-built switchbacks through fir forests for about two miles. The trail opens up into a more alpine environment with some awesome views, crossing open saddles and several rock outcrops within large avalanche paths.
Along the final mile to the lake, some sections of the trail become narrow and traverse steep slopes and slide drainages, there is exposure where the trail was cut between small cliff bands that may require caution if rocks and debris have fallen across the trail. This area is prone to wet mud and rock slides during monsoon season.
Continue climbing past the slide paths and descend a short, rocky pitch to a saddle above Lost Lake - this is the high point of the trail. Descend a short ways to the lake, where there are some great places to sit and enjoy the wilderness. Several primitive campsites can be found to the east and beyond as the trail descends another half mile to a junction with the East Fork Trail #56
before heading into the Wheeler Peak Wilderness and Horseshoe Lake. At this point you have the option to head left down the East Fork Trail to the junction of 58A and Middle Fork Road, or return via Trail #91.
The downhill return is fun. There are lots of side trails worn into the mountainside between switchbacks leading to the junction with the Middle Fork trail, most of these are steep and eroded and should be avoided to promote proper trail use. When you reach the creek crossing and the Middle Fork Trail descend back to the trailhead or, to add a couple miles to your run, cross the creek and follow the jeep road about 1 mile and 600 vertical feet up switchbacks to Middle Fork Lake