Doctor Who Classics: v. 5 - The Daemons and The Time Monster - Terrance Dicks

Even as a working adult, Sarasa Ichinokura suffers from an unshakable fatigue that's plagued her since childhood. She goes to bed one night, exhausted from her office job, and encounters a goddess(?) who informs her that her perpetual lack of energy is actually due to a deficit of "mana." Then, without any further explanation or time to process what's happening to her, Sarasa is thrust into a fantasy world!

She awakens in the body of a ten-year-old girl in a strange land filled with flying beasts and oversized wolves. She accordingly prepares for the worst, but a monster hunter named Nelly takes her in. The hitch is...now that Sarasa finally has the energy to do everything she wants to, being cooped up in a cabin is just no fun! She seeks to stand on her own in this scary new world—and that's going to mean learning magic.

It may be slow going, but Sarasa knows what to do—the best way to take her reincarnation is one step at a time!