In terms of terrain: I thought the first 5-6 day stretch of the Norte (Irun to Bilbao) was more strenuous than the
Camino Frances. The St Jean-Roncesvalles crossing was harder, of course, but the Irun-San Sebastian walk should not be underestimated, as there are lots of ups and downs, including a particularly intense short stretch. The main difference from there, though, is that while the
Camino Frances trends downward, as you move through the Pyrenean foothills, you continue to have gain/loss throughout this stretch on the Norte. The walk from Deba to Markina is a real workout.
From Bilbao to Santander, it's mostly flat, with a lot of road walking. I strongly encourage people training for the Norte to test their shoes on both offroad and pavement conditions. The upside is that you have a lot of coastal views through here. Check with local tourist offices for advice; they might be able to point out some off-road options. (Also, get some local advice on walking from Bilbao to Portugalete. The route advised in the El Pais guide, for example, strikes me as a bad idea. Follow the river north out of Bilbao and you'll eventually reach Portugalete, saving yourself a lot of kms. Along the same line, after Santander you can trim maybe 7km off the trip by taking the train-bridge between Boo de Pielagos and Mogro. Sounds dangerous, but it's actually quite easy if you take the bare minimum of care; all of the locals do it.)
From Santander to the turn-off for the Primitivo, and even continuing on to Oviedo, it's still pretty flat, rarely going above 100 meters, but you get off the pavement a bit more. The scenery is awesome through here, with the Picos de Europa to the south, the coast to the north, and the great architecture of Asturias in between.
The Primitivo from Oviedo to Lugo is a workout. None of the peaks are terribly high, but you go almost all the way back down right afterwards, so the elevation gain/loss is significant each day. I think I had five or six straight days that started low, rose to wind turbine-topped hills, and then plummeted right back down. The hardest days on the
Camino Frances are harder than anything here. But, whereas those hardest days are generally followed by an easy couple of days on the Frances, on the Primitivo it's hills, hills, and more hills.
Still, I think it's totally manageable. In particular, if you walk from Irun, you have plenty of time to build up your strength and endurance before reaching the Primitivo. And if the start seems intimidating, it's easy to set up shorter days early on. If you're on a deadline, it's easy to make up ground on all of the flat pavement further down the road.
