A good approximate calculation for calories burned walking is 1 calorie per kilogram per kilometre.
For example, a person who weighs 70kg would burn about 70 calories per kilometre walked.
This means you would ‘use up’ 350 calories (a small meal or average breakfast) in a 5 km walk.
But it also depends on your body weight, walking speed, and the terrain.
Yes, this is not much for the effort. It demonstrates calorie intake (i.e. food that you eat) reduction is far, far important that exercise to lose weight.