These are my criteria for a touring diet:
1) High calorie, low volume/weight (ie: no fruit &veg)
2) No specialist products that aren't available in every village shop (eg: energy bars etc)
3) Nothing perishable that will go off in a baking hot pannier (no fresh milk/butter/chocolate)
4) Nothing that requires specialist utensils not found in a hostel kitchen
5) Quick and easy to cook in a crowded hostel kitchen (2 pans/15 minutes max)
6) Cheap (about a quarter the price of hostel meals last time I checked)
7) No cooking and minimal washing up at breakfast time
From that lot I get:
Breakfast: Muesli, it's more energy dense than other cereals, and won't turn to crumbs when it's crammed into the luggage. A fifth of the packet, not the 50g 'portion' marked on the label, along with a whole 500cc carton of UHT milk. About 800kcals.
During the day: I start with half a loaf made into sandwiches with fish paste, that's about 1200kcal. Then I start on the biscuits, a whole packet is usually about another 1000kcal. Not chocolate biscuits unless they're individually wrapped. Sometimes I'll have pasties/nuts instead for a change, but they're higher in fat.
Evening meal: Usually something like a tin of chilli or curry with rice. Again, that's a third of a 500g packet of rice, not the 'portion' on the label. Alternatively, pasta with a tub of Dolmio sauce. About 1000kcals.
Then in the evening more bix/nuts/etc to fill up. I need about 4500-5000kcals/day.
For eating large quantities, grazing continually is better, so it feeds calories as you use them, and doesn't leave you feeling bloated after pushing a lot down all at once.
Re: Water, I carry four litres, and generally make sure I drink it all. On hot days I'll stop and buy more en-route.
Re: Bowels, I have a long history of overtraining, and that has a particular effect on my digestive system. In moderation, over-exercise causes very large quantities of very loose stools, but if pushed further, my bowel shuts down altogether. Not hard stools, just nothing at all. Unlike when I'm on tour, when I'm at home I eat about 8-9 portions of fruit and veg a day, and get about 80% more fibre than the 30g/day recommended, but I don't find it makes much difference to my bowels, one way or the other.