Good point! I’m referring to difficulty due to weather and path/road conditions. I’m not worried about the general terrain, ascent, etc. If it were summer, I wouldn’t be thinking twice about it!
The Pilgrim Office in St Jean Pied de Port will have the most up-to-date conditions of the Route Napoleon, so I consider it an essential to pop in, say hello to the volunteers, get your Credencial stamped, and get the latest information about conditions and expected weather.
Outside of a rather small subset of pilgrims who stop at Orisson for the night, the large majority will walk to Roncesvalles. For those like yourself who have some level of fitness and walking experience, it should not be aggressively difficult. Myself, I would be bored silly stopping at Orisson after such a short walk and the majority of the day ahead
. I do very much enjoy stopping at Orisson for a longer break, enjoy a fresh made bocadillo and other goodies, and to increase my fluid intake with an orange Fanta or Kas or Coke Zero.
On the walk up from SJPdP to the high point near the Col de Lepoeder, the majority of the walk is on hard-packed or paved surfaces, so they are less affected by wet ground conditions. There are some substantial sections which get soggy with deep mud if there has been a lot of rain a or still dealing with snow melt runoff.
On the descent to Roncesvalles, the less steep grade is on the narrow and paved lane. Not a whole lot of problems with wet unless one's shoes lack basic traction. The other choice down is the forest trail which is about a kilometer or two shorter, but more risky, especially for the inexperienced.
In bad weather, Valcarlos is likely to be the recommended route by the Pilgrim Office. As to energy expenditure and challenge to one's fitness, it is NOT going to be easier. Although the total elevation gain is about 300 meters less than on Napoleon, there is far more ups and downs as the route frequently gains and then looses elevation. The final stretch into Roncesvalles id done at a steeper gradient than that seen on Napoleon. It is shorter, though.