I also walked the Valcarlos route in May 2007, and totally agree with Kelly about the problem with overgrown paths. In fact, I was quite firmly told by staff at the SJPP Pilgrim Office not to walk that route as I would be walking by myself. All pilgrims were strongly encouraged to follow the Route Napoleon. My reason for walking through Valcarlos was the fact I also felt I wouldn't be able to do the walk in one day, and the town has all facilities. Once I got to Valcarlos, and stopped for coffee and food, I decided to continue and made it, eventually, to Roncesvalles.
The path from SJPP to Arneguy is very good and well-waymarked, from Arneguy to Valcarlos you walk on the road.
However, after Valcarlos, it would be better to stay on the road, especially if you are walking alone. There are Camino signs pointing to off-road tracks, but as Kelly states, these tracks are very overgrown. I tried two of them, walking 2kms on the first only to find a pilgrim coming the other way, quite agitated, because the path was in such a bad state and falling away. So I followed him back to the highway and continued on the road to the next Camino sign. Again, I followed this track for 1/2 hour only to find it petered out. As I was by myself, and had only seen the one pilgrim all day, I realised that there would be no-one to help if I had an accident, so decided to return to the highway and took this all the way to Roncesvalles. With all that to-ing and fro-ing I added an unneccessary 8kms to an already (for me) long walk.
Walking on the road is tiring, so I do recommend breaking the trip at Valcarlos. The steepness increases in the final few kms with lots of hairpin bends up to Ibaneta Pass, but the guardrails are pretty handy to rest against! Just remember to walk against the oncoming traffic, although there's not too much of that.
Trudy