It’s obviously unlikely that anyone is going to walk around the whole of Cantabria on foot! However, for those intrepid walkers, there are some interesting long distance footpaths through Cantabria, an enormous amount of shorter marked hiking trails.
The most famous is of course the northern route of the Camino de Santiago, the Camino del Norte, following the coast from east to west, and passing through many beautiful seaside towns, as well as Santander. In places it is well marked, in others it is not, but the Camino shell symbols seem to pop up everywhere along the coast. There’s also the GR99 – the Ebro River walk, starting from the village of Fontibre near Reinosa and following the course of the Ebro River (the longest in Spain), right down through Zaragoza to Tarragona. This is well signposted and even doing stretches of it is worth it as it runs along the river. Other GR routes in the region include the GR 71 from Barcena Pie de Concha over the high hills of the Saja national park and mountains of the Picos de Europa to Sotres in Asturias (via Potes), the old pilgrim route of the GR72 from Reinosa to Santillana del Mar through the Saja national park, and the very mountainous GR74 from Ramales in eastern Cantabria across to Reinosa in the south. One GR I think is worth a special mention, or at least part of it, is the the roman road of the GR73 starting in Herrera de Pisguera in Palencia, and running south to north through the centre of Cantabria to Suances. Roman road is visible along several stretches of the route, but the most impressive section is the 5km between Pesquera and Pie de Concha, which includes a long section of relatively steep, unbroken road complete with cart tracks, running up a wooded hillside, it is quite magical and one of my favourite walks (the walk is known as La Calzada Romana, and can be done without needing a car by using RENFE trains). You can make it circular by walking the Camino Real from just below Pesquera to Barcena Pie de Concha.
Rutas y Tracks (by far the most comprehensive list of walks, including all the PR and GR in Cantabria)
Cantabria Tourismo Wikiloc (Cantabria Tourist Board’s Wikiloc page)
Senderismo y Cicloturismo en Cantabria (pdf download of official tourist board leaflet)
Rutas Por Cantabria (with Google translate English option)
Cantabria Joven (slightly dated and not covering the whole region)
Top Walks Cantabrian Coast (limited selection in English)
Top Walks Liébana (Picos de Europa) (more comprehensive selection in English)
Naturea (excellent regional programme of guided walks from visitors centres around Cantabria – some guides speak English)
See the Adventure Sports section for information on companies offering further guided hikes.