Anywhere that there is a piece of the Camino de Santiago should make for good running, as it is a mixture of surfaces. (there are guide books that tell you where different surfaces are.)
Northern Spain has many mountains in the north (the Picos) and in the east, (the Pyrenees and pre-Pyrenees, which are ower-lying than the Pyrenees), all of which have many trails of varying degrees of technicality. Some better suited for running and others maybe more technical and better to hike. Spaniards frequently use WikiLocs to record hikes that you can use for routes.
Some specific places: Los lagos de Covadonga; el Valle de Tena (aragón); outside of Santiago de Compostela there are many trails in the monte (very low mountains with green forests); Gorbeia National Park (basque country); Potes; also the Spanish Greenways: http://www.viasverdes.com/
The climate for running in the summer months is better further north, southern Spain is quite hot in summer months and running outside of early morning hours can be challenging.
RE: Good running in Spain?3/13/2018 7:23PM - in reply to Don Quicksode
I would imagine Lanzarote in the Canary Islands would be interesting to trail run, but best in the winter (usually around 20 degrees Dec/Jan/Feb). Has a volcanic landscape leaving the dirt and even the sand reddish all over the island. Closest you'll get to running on Mars. Not much in the way of greenery though, aside from a few cactus plants.