Nice little loop with optional single track.
Start at the southeast end of the lake and take USFS 76 (it's marked) which parallels the lake shoreline. Gravel roads completely loop the lake. This dirt road will take you about 1.5 miles until it turns to gravel at the backside of Slaughter Gulch Campground. Stay on the gravel until single track beckons just before the access road joins USFS 89. Follow this trail for about a mile to Lost Creek. Now, either ride out onto USFS 89 and go right, to where you started, or go left and follow it around the lake, an 8-mile loop. You could also double back on the trail. A longer ride can be had (up to 18 miles) if you go to the north end of the reservoir on USFS 89 and continue taking rights down a steep hill to the Price Valley Guard Station and eventually onto US 95. This is well marked with road signs.
How to get there:
Take Interstate 55 west out of McCall to New Meadows (I know the mayor so watch your speeding!). Go straight through at the junction with US 95 and go south to Pine Ridge. Take a right at USFS 89 and follow it 3 miles to Lost Valley Reservoir. Park at the southeast end.
Lots of great camping and fishing in the area.