The only truth about fighting jet lag is that everybody is DIFFERENT and what works for some does nothing for many....I have traveled to Europe at least twice/year for the last 20+ years. This is what I do and I NEVER get jetlagged when in Europe: upon arrival to the hotel (mid-morning usually), I take a shower. Then, I TAKE A NAP FOR MAX OF 2 HOURS. I get up refreshed and ready to really get in the local rhythm. I then wait until 9-10pm to go back to bed. This has never failed me. I just came back from Eastern Europe and I was so excited to see Krakow that upon arrival to the hotel around 1pm, I just walked around for about 6 hours. Same thing, I was fine. Slept through the night every day. Who knows, I may be one of the lucky ones who never gets jetlagged to begin with!!
As far as 2 days, I would get on my way the next day, but if this is your first time in Spain/Pamplona, by all means explore. Take a map of the Camino in Pamplona and follow it from Bridge La Magadalena to the Cathedral. It is a great way to explore Pamplona while familiarizing yourself with the markers and flow of the Camino. If you can "cyber walk" the Camino in Pamplona using Google Street View do that to familiarize with the streets as it gets a bit confusing in some spots.