Etapa plácida, corta y prácticamente llana; la exposición al sol durante el recorrido es casi total, a tener en cuenta sobre todo en verano. Las localidades de Zarratón y Bañares, ambas con servicios, nos marcan la pauta. En Santo Domingo de la Calzada, cuyo campanario de la catedral vemos desde...
I remember it as a pleasant mornings walk. 19km with a breakfast stop at Zarraton and a beer at Banares. The municipal in Haro was a great starting point for the day.
I guess I could have suggested walk from Irun but I guess if the OP is flying into Bilbao then the train to Haro and tighten your boots is good enough advice
Airport shuttle to bus station, then two buses (from memory, check rome2rio.)
If you are of pension age, you can buy a train pass for discounted travel - a tarjeta dorada. Booking well in advance is often cheaper