Unless you want to get your first sello at the point where you start walking (and some pilgrims will), and depending on your route, I would rather pick one up at the ayuntamiento in San Juan de Arena, or if going by Soto del Barco, the hotel at Palacio de Magdalena (the old summer residence of the bishops of Oviedo, from the days when bishops had summer palaces) has a splendid sello. If the airport turismo has none, the pilgrim has recourse to bars and cafés for sellos--- Spain is a land of sellos. I have several guardia civil sellos and a military sello as well as one from a hospital where I took a suddenly ill pilgrim.
Another possibility (you can put this in the unasked advice category) is that church-going pilgrims will often have their first sello from their home parish.
That's a nice idea if you want to save that first bigger box for a special stamp, or if you want to get that spot stamped by your church at home, but it isn't necessary to put any stamp (or any particular stamp) in that space.
In other words, don't worry about the placement of the stamps.