As fietsbob points out, this is essentially one of the ways of St. James. You propose to go 720 km in eight days. That about 44 hours of biking, if googlemaps is anywhere near to accurate. Are you up for 5 and a half hours of biking per day?

When my Mom married my step-Dad, they were both over 60. Jim was a really enthusiastic cyclist (he built his own recumbent, which we commuted to the State Department on when he was a Foreign Service Officer.). He was also Roman Catholic, and wanted to do the Compostela de Santiago pilgrimmage. Much the same route as the OP proposes. Jim got Mom a bike. Got his (road) bike all tuned up. They practiced. Went to Spain, rented bikes ,and gave it a go. After the first day, my Mom decided that cycling the pilgranmage was not going to work for her. So they ended up doing the thing by motor vehicle. Had a great time. Point is: be flexible when you get there.