>>955875Here's the basic daily caloric use formula
9.99 x your weight in kilograms) + (6.25 x height centimeters) - (4.92 x your age in years) + 5
For you,
(9.99 × 59) + (6.25 × 175) - (4.92 × 22) + 5
Comes out to 1,580
Add in 15km of backpacking, at an average pace of 5km per hour (at sea level which in the UK I assume you are near), that's 3 hours which is about 620 calories for your age, height, and weight.
So overall you need 2,200 calories per day for 15km backpacking.
Obviously you need calorie dense and lightweight foods. Supposing you don't need to carry water and have a nearby river/stream (also it's the UK so you will have rain water), you can easily carry 4.5kg/10+lbs of food. For example at 260 calories and 68grams, you would need 1.25lbs of Clif bars per day, so 10lbs would last you 8 days. Nuts like almonds and walnuts are another source of lightweight high calorie food.
TL;DR: a bit over a week if you have a nearby water source
