I was just in Iceland for a week or so. Restaurants were a bit on the expensive side coming from the US, but not terrible. Cheap sandwiches abound there, though: at most gas stations or the supermarkets I could get a premade sandwich for $4 US or so. I largely did day tours as well, and all the ones I went on stopped at a couple of gas stations through the day for pit stops and food, and the sandwich selection was adequate. Personally,I wouldn’t stress too much.