Hi Alison Its hilly for about the 1st 120 km, part of the reason is the way the coast is shaped and when you head inland you have to climb into the hilly interior. After Gernika it settles down into a undalating rhythm for the rest of the way to Santiago, with a few exceptions. The 1st few days are not that hard IMO but that could be because I train on the South West Coast path and it has constant ups and downs. To me the 1st 3 days on the Frances are harder than the equivalent on the Norte.
Most albergues will be open from 1st July, if you have a good guide ( cicerone) you can plan a route which gets you to Santiago in less than 30 days, eroski and gronze have both good on line guides with up to date albergue lists. There is a lot of asphalt walking, so your shoes or mids need some kind of good shock absorption. The route itself does not constantly stick to the coast, you kind of zig zag backwards and forwards to it until the route gets into Galicia and starts then heading inland to Santiago. There is options in places to take non Camino coastal paths, you will usually see markings for them or hear about them from other pilgrims, Conrad Stein books( German) have bought out an English e-book for the Norte, this has lots of coastal options in it. It will be busy in July but you should be ok.
Good Luck and Buen Camino