Basically the "refresh rations equal to SPD" thing is where the body is breaking down acids and fats in the body to power yourself. The body itself is starving, and your body is breaking down whatever it can to fuel you. While exercising, the digestive system slows to a halt, so even after a large meal this can occur. This is how extreme weight loss occurs in those that are starving, as well as those that work out for extended periods of time.
This form of exercise is endurance based. It is about outlasting rather than going fastest. It's why we work so well as hunters, since we don't have to rely on conserving energy; we literally run our prey to death.
It also works in return. As long as the initial jump doesn't get us, we can run longer and farther than the thing hunting us.
By the time a character could be truly "could die from" starving, you would have already gone into monologuing/cutscenes time.
I would go into a whole speel about ATP and ADP (the chemical) and the three separate forms of exercise, including the two that come before this one, but that would be boring. I will link an article though.
https://health.howstuffworks.com/wellness/diet-fitness/exercise/sports-physiology1.htm