It is perfectly marked, you will have quite a bit of tarmac up to Erandio, then will cross the railway, leave through a small industrial state, cross the motorway over a bridge and then the real ascent begins. This is where the beautiful part of the day begins, the rest is not amazin, really.
I would consider taking a train/taxi and start directly in Erandio.
This said, once you come to the other side, the views are worth it, indeed! You will find quite a few small divertions to see lookout points and the like , do follow them if you have time. The descent onto Bilbao can be knee wrecking, but then you find the Begoña Basilica, the only church which is not flat but on a slope, curiously enough.
Buen camino! (and good choice of a restaurant, amigo!)