San Sebastian was suggested simply because it is close to being a straight line from just west of Paris. I don't know how many others will be making the same trip, how fast they will want to fly or how many hours per day they want to do.
My ideal would be from home to Chartres (sw of Paris) on the first day.
On the second day I would want to get close to the "hills" on the French side.
From there to Marugan on the third day.
Where we cross the "hills" is open to suggestion, if there is somewhere to the east of San Seb that makes Vitoria a sensible choice then I will look for a suitable place to overnight close to the line from Chartres to Vitoria. We could stop for a dip and a cuppa near to Vitoria.
Rans6....