Kinetic energy = 0.5 * m * v * v.
If you would walk at 2.5 m/s v *v = 6.25; and the equation will be 0.5 * 44.4 * 6.25 = 3.125 * 44.4 = 138,75 J.
If you walk 1 km (0.6214 mile), you will walk for 400 seconds. 400 *138.75 = 55,5 kJ. Given that the body has a conversion rate for walking of around 25%, the actual result should be multiplied by 4. Hence you would need 222 kJ to walk 1 km. (roughly 50 ((kilo))calories.
For each kilogram you gain, you will need 5 kJ extra energy to walk 1 kilometer (1.2 ((kilo)) calories). Just gain some weight and enjoy the "free" cookies
P.S.
I am skinny as well.