Most Helpful Girl
I don't believe that is all she eats cause if that was all she ate - she would be dead or visibly starving herself. I think she eats more than that but doesn't tell anybody - not saying she eats a lot more - but if somebody is burning calories and not eating calories - they can only get skinnier and skinnier - its not possible to stay the same size when you are doing that - no matter how small your original size is.
Most Helpful Guy
There is a strong correlation between her thin body and rigorous workout, and between her under-nutrition and thin body. The body is capable of adapting to any condition including extreme workout or under-nutrition. If the details you mentioned in your post are factually true, she is visually unhealthy by conventional medical assessment.
It would have been helpful to provide details of the foods or meals and their quantity she eats daily. If she has been eating on a caloric surplus for three or more days per week, it is perfectly reasonable to still feel highly energised after a gruelling running.