Hiked the Valcarlos route last October. I'm 60, a 5k runner, but pretty creaky. Starting in SJPP is pretty cool. My wife and I stopped in Valcarlos the first night, a bit jet lagged. The second day from Valcarlos was a good challenge due to the big change in elevation (check out the elevation map in
Brierley's book - it is not a walk in the park by any means). Valcarlos has a god bit of road walking and some traffic, but we thought it was interesting. My wife developed blisters between her toes, using shoes she had hiked 70 miles of camino in the previous spring - we figure it was the pavement, not the shoes; didn't have problems after that. An 80 year old guy from sweden hiked with us - nice and slow - I believe it was a big challenge for him, but he toughed it out and made it.
If we were to do it again, we would go over the mountain, stopping in Orrison, probably sending some of our gear ahead.
Option 1: Send non-essential gear to Orrison( if they will take it), hike to and spend the night in Orrison (reserve ahead). Send gear the next day to Roncesvalles, but maybe take transportation as far as you can from Orrison. When we arrived in Roncesvalles, we were surprised by the number of folks that had been dropped off on the road part way up the Napoleon route, and hiked over with day packs containing food, water and warm/wet gear. Sorry, I don't have any particulars on how they got transportation, the map in
Brierley gives you an idea how far the road will take you. Do whatever way works for you and your physical condition, and don't be concerned with what other pilgrims think - save your body for the rest of the journey.
Option 2: less stress. Same as option 1 but skip Orrison and take transportation to the last spot where the road leaves the Napoleon trail.
The 2 days of out and back you described sounds nearly as challenging. I believe the elevation change from SJPP to Orrison is nearly 700 meters (2200 feet - a lot), and coming back down can be worse than climbing.
In any case, you'll get lots of opinions!
Buen Camino!