Where does heat that warms your body come from

The heat in our body, known as body heat doesn't come from somewhere, we generate it through processes. Body heat is generated during metabolism, a process where we obtain energy from the food we consume.

- As a rule, energy, is heat.

Therefore, when you say, "I lost a lot of energy running", is the same as saying "I lost a lot of heat running", because while you run, your body needs energy, and the excess energy is released into the environment, in the form of heat.

Hope it helped,

BioTeacher101