Alternatively, plan to get as far as Bayonne on the day of arrival. Stay at a hostal or small hotel near the train station. There are several very good places to stay. Search here for recommendations (hostals in Bayonne), then consult booking.com. Many of us find this site particularly helpful along the
Camino Frances.
The next morning, take the first or second train to Saint Jean Pied de Port (SJPdP) for the day. I believe there are four trains per day from Bayonne to Saint Jean Pied de Port. I found the times from Bayonne to be 0745, 1110, 1455, and 1806. Check the schedules here:
http://www.ter.sncf.com/aquitaine
That places you in SJPdP in plenty of time to get to the Pilgrim Office, check-in get a Credencial if needed, get your scallop shell, and any advisories regarding the Napoleon Pass and the Valcarlos Route, and to find overnight albergue lodging, and start the next day.
However, my personal recommendation, given your arrival at Biarritz, is to make a reservation in Bayonne for one night, and again at Saint Jean Pied de Port to stay in a hostal or hotel in SJPdP for one to two nights, so you have time to acclimate better, and tour this charming and wonderful French town. There are shops to obtain any last minute items you need both in town and "on the flats", to the left coming out of the train station. You can get a map locally. I use booking.com top make advance reservations here. Again, use the Forum search engine to search for "hotels Saint Jean Pied de Port."
This town has been sending pilgrims on their way for over a thousand years. Attend the Pilgrim Mass the night before. It is a ceremony that reaches back a millennia and should not be missed. All faiths are welcome. The Pilgrim Office , at #39 Rue de Citadelle will have the specific information you need.
I hope this helps.