Hi Brian,

You'll be crossing many towns (Toulouse, Auch, Pau, Oloron, Jaca, Sangüesa, Puente la Reina) where there are many shops and therefore nearly any food you may dream of.  There are also a few shops in big villages (Pibrac, l'Isle Jourdain, Gimont, Marciac, Maubourguet, Bedous, Monreal). Otherwise, small villages may have or not a basic supermarket / grocery / bakery.
Don't worry, you'll find dry fruits (personally, I love dry apricots) and nuts along the path. Not in every shop, not always the ones you prefer, but you'll not be starving provided that you build up small stocks prior to starting your stage. Same remark for dry sausages, ham, patés, cheese.  The most common bread is white french bread (the so-called 'baguette'), not the ideal one as far as taste and energy are concerned. But many bakeries and supermarkets also sell ameliorated breads or even sometimes whole-grain breads.
The key point for you will be to identify shops 1 or 2 stages in advance with the assistance of guide-books or local people / other pilgrims.  Besides, be careful with opening days and hours, they vary from one place to another! Closing days are often sundays and/or mondays, but not always! Small shops often close 3 hours for lunchtime (people are used to a nap during hot hours! A good habit also recommended to hikers!), sometimes from 12am to 3pm, sometimes from 1pm to 4pm.
If you start at Pibrac and go to Puente la Reina, your hike may be around 18 days long. Eating plainly as you said may prove quite handy but you would benefit from having a good dinner from time to time. Hiking along the Arles route is not much energy consuming but if you don't eat properly and enough you may start feeling a bit weak after 10-15 days. This region is well-known for its gastronomy standards. Why not enjoying local specialities as well as building up a strongest body.

Blanco