If you'd never run before, you'd feel like crap after running 2 miles. Nonetheless, your body is designed to run; if you have a typical human body, take it slow and keep training, it won't be long before 5 miles is totally do-able. (Obvs individuals will have specific limitations.)

It's the same with fasting: If your body isn't used to fasting, you feel like crap; nonetheless, your body was designed to fast. Take it slow and keep practicing, and the rust will work off all those unused chemical pathways handed down by your ancestors. (Again, if you have diabetes or hypoglycemia or something like that, maybe fasting isn't for you.)

Because your body isn't used to running off its own fat. If you're on a keto diet you can effortlessly skip meals, but the initial switch to a keto diet is hard for the same reason.