I have just returned from the Norte. I had read that there was much hard surface walking, but sort of dismissed it as general pilgrim whingeing. This was my fourth Camino, including the Portuguese which is also described as having a lot of road walking. But you should believe what you read. I was really shocked at the amount of hard surface walking on the Norte, it felt like way, way more than any of my other caminos. And there was also less walking alongside the coast than I was niaively expecting. All this misconception was because I don't really study guide books, rather take it as it comes.
If I had bothered to properly research and know in advance the above information would it have changed my mind about walking el Norte? Not one bit.
A Camino is what it is, and we pilgrims just take it as it comes (with the option of bussing ahead every now and then - not an option that I take).
There aren't so many stretches on busy highways, more on quiet country roads with little traffic, and through the cities that you will pass. Be sure to have really good cushioning in your footwear.
When you do hit the beaches and the cliff tops it will make up for all those hours of plodding on asphalt.
You are welcome to take a look at my blog - I posted every day live from the Camino and mention what the walking surfaces have been like...
https://magwood.me/el-norte-ingles/