Gernika to Bilbao

Not to equivocate, but a lot of it depends upon your conditioning and the state of your body at that moment. Those first few days on the Norte can take a toll--they're awesome, but they also have a lot of ups and downs. How are your knees? Have you walked 32-35km in one day before?

When I take students on the Norte this summer, we'll do what you're hoping for--Irun to Bilbao in six days. We'll also train pretty hard.

BTW, if you do this, I'd recommend against walking all the way to the pilgrim's albergue in Bilbao. It's out of the center all the way on the other side of town. There's a new hostel in the old town, the Ganbara Hostel (along with plenty of hostales), that will catch you as you first arrive, shortening the day to around 32km. That difference can really help you to enjoy Bilbao after the walk.

As was said by previous poster, it depends on the shape you're in. I had a long day walking, but made it in. I had a brief siesta after lunch in Lezama, and enjoyed a bench in the plaza. After that, I hoofed it into Bilbao, and stayed at a pension in the centre, not far from the Bellas Artes (which, as a gallery, I preferred to the Guggenheim, but YMMV).

Chris
when I did it a couple of weeks ago at least four of us walked the whole way, but quite a few hopped on the bus around halfway.
A few of us stayed in Portugalete the next night so that Castro Urdiales was a manageable distance, as we knew the albergue in Pobena would be closed.
I pretty much had a rest day in Bilbao and then set out for Portugalete about 3pm. It took a bit more than a couple of hours to get there, following the right bank of the river. The Vizcaya bridge costs something like 35 cents to get across (and it's brilliant) and there's a board map of the town by the exit point. This was handy as it showed the street of the pension I was aiming for (which I'd found in the Cicerone guide). But if you're not wedded to walking every step of the way you can always catch the metro to Portugalete the morning after your full rest day.
cheers, tom