I assume you will be driving as to do that by train is not possible.
San Sebastian to Pau is about 2 hours drive, Pau to Lourdes about 45 minutes, Lourdes to Bayonne about 90 minutes and Bayonne to San Sebastian about 30 minutes. A long day behind the wheel, lots of money to spent on fuel and highway tolls, but do-able I'd say.
You don't say when you are planning this trip. What happens in Lourdes is depending on the time of year.
It will be a long day but traffic should be fine at that time of year. From Pau don't bother going back on to the motorway to drive to Lourdes, stick to either the N117 (and then D940) or if you are confident about driving the smaller Route de Nay (D936).
Once in Lourdes I would park on the outskirts and then walk in as the streets are quite busy with pedestrians.
If you want to get the "spiritual effect" of Lourdes, you have to give it more than a few hours. Are you going for the baths? That takes a bit of time as you have to wait in the line up. The Candlelight Procession is the most moving but it is at night. If you don't have that much time, I suggest you save it for another time and do Biarritz and St Jean de Luz. Remember that to enjoy your meals, you have to invest at least a minimum of 1.5 hours.
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