This off-road route takes you straight to the iconic Orellán lookout (Mirador de Orellán), the spot for the best views of Las Médulas. You will also pass the galleries (open for visits), the ancient tunnels that pushed the water through narrowing spaces at increasing speeds so it would blow up the hills and disgorge the gold. From the mirador, you can either go down to the village of Las Médulas, or bypass the village entirely by carrying on to Puente Domingo Flórez. The alternative is about 11 km with a total of 475 m elevation gain.
The official camino route from Borrenes up to Las Médulas has about 300 meters of elevation gain, but remember that if you want to visit the Orellán viewpoint, you will have to walk those additional meters of elevation at some point!
Saturno and Soledad (AKA Marisol) in the Centro de Turismo Rural Cornatel can give you plenty of information.
The alternative is fairly well marked, but a GPS should be used. Going this way has the big advantage that you will arrive at the best viewing point in Las Médulas before you get to town, leaving your afternoon free for other things. There is more elevation gain than on the official route, and it is 4 km longer as well.