Burgos to Santander?

What about 'winging it' north following the N623? It is roughly 160 k or a 5 day hike. Also check what trails are available from Burgos/Santander on wikiloc.

The former Santander-Mediterráneo railway was being adapted to be converted on a Vía Verde (Green Track) from Burgos onwards. Works have ended in some areas and all the route is expected to be adapted this year. However, that railway ends at La Engaña tunnel (between Burgos and Cantabria) because the railway from that tunnel to Santander never was made so from that point (or a bit early if they didn't reform the tunnel for the Vía Verde because the tunnel was in really bad condition after decades without use) you would need to take public tranport to Santander (e.g. from Vega de Pas) or to design your own route or to walk on the shoulder of paved roads till Santander.