Just FYI, because of the steepish climb the first day, I usually make it the 8 km in about 3 hours. I take my time and wait for 'tweedle dee' and 'tweedle dum' to catch me up. Those are my personal nicknames for my lungs...
I frequently have to stop and catch my breath, figuratively for 'the boys' to catch me up as I do this walk, and a couple of others at the beginning of any long Camino. It can feel like you are hacking up a lung at times, especially if you are a pear-shaped, "chunky monkey" like me...
Preconditioning is important, but not absolutely vital if you are in basically good shape and have no chronic issues that will cause you to fall down stone dead with this sort of exertion. If you have such issues then you need to think about alternatives to walking this first stage. You can get a taxi or shuttle ride to Roncesvalles, or start at Pamplona.
It's all good, and it is YOUR Camino. I know people who have started touring Saint Jean Pied de Port for a day or two then caught a taxi to the 10 km point, 2 km AFTER Orisson, and walking the rest of the way over the Pyrenees into Spain from there. The views are still amazing.
Generally, and for most pilgrims, the first week or so is training, on-the-job, so to speak. After that, your body starts to get used to doing something abnormal daily. After about two weeks your body is conditioned to the pattern. You can search the forum for discussions regarding the emotional or metaphysical stages of a camino.
Hope this helps.