According to Gronze, that 37km is the lenght of the new route. They say the 'bridge' is closed for walkers.
"En 2016 hubo una importante modificación del itineraro: Desde Oruña el camino oficial se dirige ahora directamente a Mar sin subir a Mogro, reduciendo considerablemente la longitud de la etapa. Ya es innecesario cruzar el río Pas por el puente ferroviario para acortar la etapa; además, cruzarlo a pie, como lo hacían muchos peregrinos, está prohibido por motivos de seguridad. "
In 2016 there was an important change in itinerary: Since Oruna, the official camino gor directly to Mar withouth passing by Mogro, considerably reducing the lenght of this stage. it´s not necessary anymore to cross the Paz river by the railway bridge to shorten the stage; actually, crossing it on foot, as many pilgrims did, is prohibited for safety reasons".
Unless you are used to those long distances, I really think it is quite a challenge to walk 37km on the first day, and in a region where is is all asphalt and highways. Maybe consider the train out of santander, or break that stage in two. You can see all accomodation here:
https://www.gronze.com/etapa/santander/santillana-mar
I have not walked that route, maybe
@trecile can help as she was there recently.