I have walked from Ponferrada to Las Medulas, but that was because I walked the Camino de Invierno, which goes from Ponferrada south of the
Camino Frances. You couldn't walk there and back in a day, but you could walk there, spend the night there (I can give you names of places to stay) and walk back to Ponferrada the next day. You would also have the advantage of being able to visit the pretty amazingly perched castle of Cornatel, which is along the way.
If you are interested in the Camino de Invierno, it is a beautiful, well-marked, solitary alternative to the
Camino frances. There's a section of this forum dedicated to it, you can get ideas on stages, etc.
I think Las Medulas is a UNESCO world heritage site. What I learned in the museum I visited there is that it's what left of the mountains after the Romans blasted water through them and made the mountains explode and give up their gold. So these are essentially mountain innards. In spite of such an ignominious origin, it is an imposing kind of site. You can walk up to a ridge on the southern side and get an amazing view from afar, and you can also take some of the many trails through the area, weaving around the stumps that are left.
There is some good elevation gain, two major ascents from Ponferrada, one up to the castle, then you descend and then go up again to As Medulas. So it's not an easy walk, but it is a great walk! Buen camino, Laurie