In my household we have vivofit and fitbit devices. My Garmin vivofit has a long battery life (months) while my wife's fitbit lasts a few days at most. Model may be important, but Fitbits do need to be recharged. The vivofit seems pretty accurate on its step count. On the flat, that is great, but in more hilly country the conversion from steps to distance will be less accurate. My recommendation would be the vivofit for the camino of these two.
The disadvantage of activity trackers like the fitbit or vivofit is that reset daily at midnight, and count steps continuously. They cannot be reset or zero'd in any way during the day, so if you are wanting to check distances, it means keeping track of the distance at the last point. I think this would be a pain in the ... .
As for watches, I have the Garmin Forerunner 15, which is a not quite bottom of the range sports watch. Battery life using the GPS is poor - a few hours at best in my current experience. I would doubt that it would last for even a moderate walking day on the camino used as a GPS, and would need to be recharged every night. Here again, different models may give different results, but I wouldn't expect the GPS on cheaper sports styles would be much better whatever the make. Hiking watches seem to have much longer battery life (up to 50 hours as a GPS) but at considerably greater expense.
I carry a standalone GPS, at this stage the Garmin eTrex 30. If you don't want to carry a phone, you are not likely to want to carry this. Two AA batteries last a couple of days, and can be found almost anywhere. Full GPS functions, etc, etc. I use it in preference to my current and previous smartphones. There are a few GPS apps that seem reasonable, but battery life is rubbish on my current phone, and my previous phone never reliably detected enough GPS satellites to be workable.
On balance given what you have said, a Garmin vivofit activity monitor might be your best option for tracking distance (by counting steps) noting that it is still not ideal for measuring intermediate distances.