the difficulty of your walk in order to burn more calories is to increase the incline.
another way to burn more calories and fat when walking is to increase the speed.
exercise like push-up, Triple Extension, High Knee Skip
Begin the movement by taking a long stride and stepping forward with one leg
kick your heels back towards your glutes, flexing your hamstrings with each rep.
Keeping your torso upright and your core tight, start to march your knees up above your hips back and forth.