Yes thats much better than trying to rush from Biarritz. Its actually nice to relax in SJPDP because its actually one of the most picturesque gorgeous towns on the camino.. if you take the early train.. (i think from memory there are just 2 trains per day) then you will have all day to explore SJPDP and mentally prepare for the early start the next morning.
Worth noting that most of the hostels dont open their doors until mid afternoon.. so you might end up waiting with your pack for the doors to open.. some places will let you store your pack until later when the beds are made and you can be allocated a bed. SJPDP is a laid back kind of place and they dont often keep to any schedule.. shops and hostels open when they feel like opening
and close at strange times also. We ended up spending an extra day there (2 nights) as its such a lovely town with an old castle ruins to explore and so on.
Also worth noting is that the first day leaving SJPDP is one of the hardest days early on in the camino, so start nice and early
We got one of the last beds at the monastery at Roncesvalles, because we were quite slow. We never booked ahead and just went on faith and it always worked out.. sometimes just some floor space to sleep on..
There is something magical about just walking without having every day and every accomodation booked in advance..
Trust and faith will always find you something
Buen camino!