I actually did this in reverse and didn’t quite make it to La Capilla after a crash in deep sand sapped my detour time to meet back up with Jess on the Inter-American. So if you do this loop, let me know how it was.
Riding south on the Inter-American, you’ll come to a roundabout at Huacatambo and its two small lakes. Take your first right to go southwest towards the coast. At La Capilla downtown, make a left and follow this nameless route on a crossing of the Río Nepeña. From here make your way through some deep sand back to the Inter-American.
Ignore the mapped route (here) because Google just won’t cooperate. I’ve marked on the endpoint as where you want to rejoin the Inter-American. Use the satellite imagery because not all of the route is mapped.