Where I live, Costa Rica, we do not have a great choice either. Even more so 3 years ago, when we first started the Camino. I bought a pair of men's Merrell hiking boots. In fact there wasn't much else to choose from. Some Reebock, but not waterproof and still always for men. I ended up with terrible blisters on my heels and horrible heel pain too. That year, we only walked for 8 days (Roncevalles to Nájera) due to time limitations. Frankly, I would have had to stop a day or so in any case, had it not been that we had to return home. So - once back in Costa Rica, and determined to carry on the Camino from where we left off, the first thing to do was to find a new pair of boots. I chose a pair of Timberland's (mens, but much narrower fitting and a half a size smaller). They are fantastic. Last year I walked for 30 days without the slightest problem.
To sum this up, the Merrells were very wide and I think that my feet simply slopped around in them.By the way, I still wore the same socks (Thorlo) on both walks. Most certainly you need to take a larger fitting boot than your usual shoe size. Also you should try them on with the socks you intend to wear during your walk and, if possible do your buying at the end of the day when your feet will be slightly larger. You should not be able to feel your toes touching the end of the boot. As Portia writes, there are heel inserts. I now use them, which apart from cushioning the heel, they also stop the heel from sliding around. My husband Adriaan used his Merrells on both occasions and didn't have the slightest problem. (But then that was a man's foot in a man's boot)! They have now worn out and he has recently bought a new pair, same model, for our Camino this September.Anne