There is a train ticket office in CDG. Clerks speak English and are used to foreigners.
In Wednesday, getting a ticket for a train is not generally a problem. To Bayonne, they leave from Montparnasse "gare" or station. Remember, there are many stations in Paris.
Overnight TGV trains require reservation. I see a Wednesday overnight train leaving Paris Austerlitz at 21:52, with a connection at Dax 07:41 and arriving at Bayonne at 08:26. Then, a 11:10 train to SJPP, arriving an hour later. Quite convenient.
I took this train many years ago, in my first walk to Roncesvalles (only it was a direct one to B.). They are quite comfortable, I arrived in time to visit the cathedral, have a stroll in the pedestrian street of Bayonne, a cafe au lait and a pain chocolat for breakfast. Very laid back, after the bustle and noise of Paris.
If this is your option, you can take from CDG "Le Bus Direct" (formerly Cars Air France) to Gare de Lyon, and walk 15 minutes to Austerlitz, the other side of the Seine. Store your backpack in the lockers ("consignes") outside the station and presto, you are set for a nice day walk in this marvelous city.
Buen camino!
, Caesar
