Bilbao finish after 3 days - where to start ?

I'd strongly recommend the following:

- Get the bus straight from the airport eastwards to Zarautz (otherwise you'll have to go into the centre of Bilbao and navigate your way around the busy Termibus to make another bus connection). There's a direct connection from the airport with Lurraldebus, you can check for times here.

- I'd recommend walking from Zarautz to Gernika (the site of the bombing famously portrayed in Picasso's work of the same name). I suspect you're definitely going to want to experience walking near the sea, and the "three days before Bilbao" are mostly inland. By starting from Zarautz you'll also get to see the famous "Flysch" rock formations, which are really worth the trip in themselves.

- You might get a wee bit further than Gernika in three days (if you're very fit), but from here on you're basically just walking through the outskirts of Bilbao. So I'd definitely catch a bus for this last stretch.

I would definitely suggest you skip walking the last 10 or 15 km into Bilbao. Its probably the most boring stretch of pavement on the entire del Norte!