UpInTheAir,
It all depends on your travel plans, and where are you coming from. There is a small airport at San Sebastian, but only with Iberia flights to/from Madrid or Barcelona. Iberia is in Oneworld, so you could probably also find some combination with AA.
Anyway, Bilbao is also near (about 45 min. drive), and there you have direct connections with London, Paris or Milan (but not with the US). Driving from Madrid to San Sebastian would be around 4 or 4.5 hours (a quite good motorway), and more or less the same from Barcelona (perhaps a little bit more).
San Sebastian is wonderful a the Maria Cristina a great hotel, so I hope you'll have a very nice time there.