In my guidebook for the Frances (John Brierly
) stage 18 does have two options. Just past Sahagun there is a split. (1) To the right to Calzadilla de los Hermanillos takes you down an old roman road with views of fields and is a peaceful walk to a small village. I took this route in 2013. I loved the little village and made fond memories there however the roman road was painful on the feet. Leaving the next morning expect to continue on the roman old.
I can't speak of the other route (2) that veers to the left the Royal French Way, Real Camino France towards Bercianos del Real Camino but the guidebook says it's alongside a little use asphalt road parallel to the autopista.
Is this the stage you are asking about??