In answer your question about miles per hike....
I think about trips in elevation gain and the amount of weight I'll be carrying. For example, a five mile hike carrying 20 pounds is not a long hike if you're not climbing. On the other hand, if you're packing and getting 2500 feet or more in elvation gain, 5 - 6 miles is ambitious. I have a solid sense of how much food meets my energy needs for different kinds of hikes, which is 3 meals of varrying caloric density and 2 snacks.
I'm leaving for Escalante tonight (yay!!). My trip is very easy and looks like this.
Thursday: day hike from a base camp, 14 miles round trip. Carrying map, compass, water, food, sunscreen, hat, headlamp, knife, wind shell for the 40F drop in temp once the sun sets, and TP.
Menu is an apple, 3 oz cheese, 3 oz salami, 3.5 oz smoked salmon, 1 pita, 4 oz hummus, 1/2 bag chicken jerky, 2 bars, 3 chocolate squares, trail mix. I'll have dinner at base camp.
Friday: 7 miles with no clmbing, packing 25 lbs (no water source, walking in the desert, high temp at 90F, so I'm carrying 7 liters).
Menu is coffee, 1 dehydrated egg & bacon breakfast, 1 dehydrated pasta lunch and 1 dehydrated soup for lunch, 1 dehydrated chicken meal and 1 deydrated green bean for dinner, and 2 bars, 3 chocolate squares, trail mix for snacks.
Saturday: camping near water source, repeat of Friday's distance, effort, and weight.
Menu is a repeat of Friday.
Sunday: 3 miles.
Menu: dehydrated breakfast, 2 bars.
I may not eat all the food, but it's absolutely critical in my opinion to have extra food and water in case I get stuck out there for a day or two. Dehydration is the number one killer in the desert.