This road starts at the La Plata Peak Trailhead right off Colorado 82 - which closes in winter. If you are running the summit, park here and run along the road - don't assume you can park where the trail branches off.
From the trailhead, you'll head south and come to a bridge in a few hundred feet that crosses North Fork Lake Creek. After about a third of a mile, you'll see where La Plata Peak Trail
heads off to the east. The forest road continues south/southwest along the South Fork Lake Creek for another half mile. At this point, the road turns into Forest Road 399.