I think the vocabulary here is a bit misleading. The camino between Molinaseca and Ponferrada is not a highway, nor is it along rural paths. It is a paved path adjacent to a tertiary road between the two cities. In North America we would call it a sidewalk, you may know it as the pavement. What is certain is that it is paved. With the hills to your south and the river to the north, there are no alternatives to this path. It is not the greatest 7.5 km to cover at the end of a long day descending the mountain.
From Ponferrada the situation improves greatly. There are a few stretches of city walking (out of Ponferrada, through Cacabelos), but the majority is gravel roads and footpaths. The scenery shifts too, and is a 50/50 balance of city and vineyards.