If you are staying at Granero del Burro and want an easy, but interesting walk, then this is the perfect answer. You can make it as long or short as you want but the full distance between San Tirso de Abres and A Pontenova is 12 kilometres (so 24km there and back). It follows the route of the old railway line that opened in 1905. It used to take the iron ore from the mines near A Pontenova to be loaded on to the boats in Ribadeo. You can still see the 'cargadero' in Ribadeo.
The walk follows the course of the River Eo and has 6km in Asturias and then 6km in Galicia. If you walk the full distance, you go through six tunnels which are lit by lights powered by solar panels. They vary in length from 60 to 230 metres and are great fun! It is best to start in San Tirso and you can park by the bridge that crosses the river. If you don't want to walk, then you can always hire a bike and cycle the route. Bikes can be hired from Eo Activo which is just by the car park.
The walk (or bike ride) is easy as it is flat and a good surface. Towards A Pontenova it become tarmac. There is a Salmon Ladder along the route as well as a couple of suspension bridges, a canal (the opposite side of the river) and a small Hydro electric power station. If you are tired when you have finished then you can take a dip in the outdoor swimming pool in San Tirso, which is free. It is open in July and August.