Hi vanjohn,
Congratulations and buen camino!!
You can get a train from Biarritz that leaves around 8:00 in the morning or maybe a bit earlier and there's a youth hostel close to the train station (or lots of reasonable two star hotels in the old town also within a couple of kilometers). You'll have to change trains in Bayonne, which is only about a ten minute ride from Biarritz. You do have time to get the next train if you move quickly. Check out the signs to see which bay the train to SJPP is leaving from because you will have to change bays and possibly even cross to the other side. You'll get to SJPP about ten or ten-thirty with lots of time to go to the pilgrims' office, pick up some food for your journey and even have lunch if you want to.
I have always headed up the mountain and stopped at either Huntto or Orisson for the night. It makes the next day much easier (only 17 km).
If you want to stop at Orisson (about 10k up the mountain), you'll need to make a reservation ahead of time. You can make it now from their website
http://www.refuge-orisson.com (click the contact button). They are almost completely booked for most of May already. The albergue at Huntto doesn't usually need a reservation this early in the year but the pilgrims' office in SJPP will phone up for you to make sure they have room if you do decide to stay there. Of course you can also stay in SJPP for the night All of the places mentioned are a great place to meet your first fellow walking pilgrims.
Cecelia