What Do I Eat When I'm Backpacking?

The cardinal rule here is: maximum calories, minimum weight. This means you may have to take high-calorie foods that, as a health-conscious outdoors person, you'd never eat in "real" life. Consider pemmican, which the Indians of the northeast United States ate on hunting expeditions: berries and dried meat suspended in animal fat – maximum calories, minimum weight. Also, foods high in sodium will replace the salt lost in excessive sweating.
Granola mixed with Grape-Nuts is a high-energy, compact breakfast, or you can just eat a couple of energy bars. Powdered milk, coffee and cocoa save weight. For lunch, choose durable crackers, breadsticks or bagels over crushable bread.  Hard cheeses such as Gouda, mozzarella, or Swiss and hard-boiled eggs hold up for a couple of days out of refrigeration. Ignore your impulse to shun high-fat and –sodium salami and beef jerky.  Base dinners on quick-cooking rice, couscous, Ramen noodles or pasta. Tinned meats aren't too much weight to haul for shorter trips. There are many varieties of dehydrated vegetables now, but cabbage and cauliflower hold up well on a long weekend.
Purify water with a commercial filter or the old-fashioned way: iodine, in pills or liquid. For liquid, two drops in a half-liter and four drops in a litre suffice, then shake the bottle. Let stand for a half-hour (or longer if the water is very cold).
Get rid of the iodine taste with the pills' accompanying "neutraliser" – a fancy version of plain old ascorbic acid, vitamin C. Scrape or nibble a corner off of a pill into the bottle. Give it a good shake, and the iodine taste is magically gone. But you must wait the full half hour to add the C, or the iodine's purifying effect will be negated.