"Mortals" vs "Human Beings"

The word mortal is used to describe people in contrast to immortals, beings that cannot die, such as gods or similar entities in works of fiction or myth. You wouldn’t use the word mortal to refer to humans in any context except one where humans were being either implicitly or explicitly compared to gods.

The book title Running for Mortals is kind of a joke, implying that unlike other books containing advice and instruction for runners that are only applicable to those with godlike ability, stamina, and discipline, this book can be used by mere mortals.

The word "mortal" means "subject to death," meaning if one can be killed, one is mortal. It does not apply specifically to any one topic.

The title Running for Mortals is a nod to the notion that hardcore distance runners are "superhuman."