There is a Decathlon store in Leira, which I believe is ahead of you. They carry Salomon and Merrill, plus their own house brand Queucha. It provides excellent value for money, if you can find a style and size to fit your needs.
If you have internet access, try
www.decathlon.pt. At the top of the page, there is a link to Decathlon stores. You click on your map location and it will list stores in the region, province, or district. For Tui and Spain generally, go to
www.Decathlon.es.
I checked the Keen Footwear web site. No joy there for all of Portugal. The distributor for Portugal is in Spain.
I specifically tried to locate a Decathlon at Porto, as it is in your "line of march," to no success. But there is likely a Decathlon store in a nearby suburb. It is worth an extra day at Porto, and a bus or taxi ride to where the store is. Local folks at Porto will know a sport store to refer you to.
In my experience on the coastal route last year, I found the most helpful Portuguese phrase to be: (phonetically) "fala ingles?" It means, do you speak English. You will be very pleasantly surprised at the results. So, you will have no problem finding a sports store to locate suitable footwear at Porto, assuming your present footwear will last that long.
In the meantime, use duck tape to bind the toe box if need be. It does work as a short-term fix.
I hope this helps.
