...As already mentioned drink LOTS of water. I find myself drinking between 6-8 liters a day when walking the camino... some days more depending on the heat and the kilometres and no I do not have "to go" all the time. The body is an amazing machine... and like all machines it also needs its fuel. Water is amazing. Drinking lots of water you will not dehydrate... once dehydrated it is quite difficult to get back to normal... Also this works wonders for the muscles - you wont have that much pain from walking if any. - Water also makes the digestion work better...
As for fibers... Eat nuts, raisins, bananas, veggies... You can ALWAYS ask for boiled veggies... and another good thing: ask for a bowl of lentejas (spanish lentel soup) it is absolutely lovely, tasty and filling.... and VERY healthy. It gives you energy.
There are also "snack bars" some more healthy than others... but all spanish supermarkets have them... (i believe they are called granola bars in english).
If you are a vegetarian like me... (well ok I do eat fish)... you can still survive the camino, though spanish restaurants tend to put meat in everything. I always remove the meat from the soups... - ham is not considered as meat in Spain... sometimes when I ask... "¿lleva carne?"
does it have meat? They usually answer me "No! lleva jamon york" (no it has york ham).
Also moving all day seems to keep my digestion in good order...
...and now a "secret" of mine which I will share with you

Even though I tend to be a weight freak when it comes to my backpack there is something I keep in there during most of the journey... something I love to carry and something I cannot and will not live without. I carry a small plastic cup, a plastic spoon and some "oats" for cooking my self some oatmeal in the morning... this can easily be done without a stove. Just put the oats in the cups... pour over some boiling water... let it sit for a while... and after 2 minutes it is ready to eat. - this keeps my stomach going. - it is weight I am more than happy to carry.... and when ever I get to León or another "big" city - they usually have a health shop.. and there you can get them... some mayor supermarket also have them... (here in Spain they are not so common).
Enjoy
