Annie,
I have only walked the Valcarlos route but did it 10 times. You can read my comments re these adventures in my
Camino Gazetteer.
The Pilgrim Office in SJPdP will give you a schematic map showing both routes.
Within SJPdP both caminos follow the rue de La Citadelle which becomes the rue d' Espagne after crossing the river and then the rue de Saint-Michel which will start to climb. At the first y junction the marker for the Valcarlos route turns right to follow along the Chemin de Mayorga and the marker for the Napoleon goes left.
There is off road walking but I would NOT take it at dawn/dusk. It would quickly get dark and dangerous in the wood.
When the path diverges off the road into the woods; I always followed the roads parallel to the path. The roads are fine but do walk on the left-side verges facing traffic.
If you walk at dawn/dusk wear something bright and csrry your light to flash towards on-coming traffic.
Between SJPdP and Valcarlos there is only one place for food/water/petrol near the old frontier; between Valcarlos and Roncesvalles there is NO place for food nor water. Hence you must be prepared!
Will you have a reservation in Valcarlos ? The municipal albergue is great and you can reserve there.
Happy planning and Buen camino!