Great choice of the Aragon! Beautiful route that is overlooked and pretty empty most of the year, though they are working hard to change that.
From Paris, you’d take a TGV train to Pau (where you could start walking) and then another to Oloron-Sainte-Marie (where you can start walking), followed by a bus to Somport.
If you start lower down in France and walk the Camino Arles from Oloron-Sainte-Marie, you’d have 2 days up a lovely, verdant valley which would include a monastery stay! I highly recommend that. Other options are starting in Lourdes (another pilgrimage destination) or SJPdP which is two days walk from OSM. In short, so many choices, but all fabulous ones!
Just remember that Somport is actually higher than the route from SJPdP and gets snow from October through May.