Spain is criss-crossed with old pilgrim routes many of which are in use to today.
The Northern Route to Santiago will take you from Bilbao to Irun (just below Hendaye).
The routes day to day can be quite rough on the feet or very easy, very scenic or very ugly, very pleasant or the worst days walking you've ever had..
Walking in June and July can feel a little crowded too, although by the July 10th things will quiet down a bit.
The route is way-marked, route maps and guides are available and most are free.
If you want a roof over your head and a shower at the end of each day there is plenty of cheap accommodation, sometimes that can be free too.
Post pics here when you get back.
Link to a page with a map.
Link to the same site with some pics.