Yes, the walk from Irun to Bilbao was tough. I was so exhausted on the day I walked into Bilbao, that I took a rest day. BUT, that first week was so worth it. The harbors and the bay towns, and the beautiful mountain sides of the Basque country were stunning.
As a side note, I arrived early in Irun, and decided to walk into Hendaye, France, right across the Santiago bridge, for a French meal before starting the next day. I got there too early, and all the restaurants were closed. The church bells were calling the locals to Mass, so I joined them. At the end of Mass I asked the priest if he would give me a stamp on my credential, the first of my journey, before walking back to Irun. So my walking pilgrimage actually started a day earlier than I intended, across the border in France. If you’re starting in Irun, consider it. It’s fun to say that you started your Camino in France.