Injuries from carrying another person?

In a book I'm writing, there's a segment where one character (female) has to carry another unconscious character (male) to safety. He outweighs her by about 20 pounds. The terrain is a relatively level forest, transitioning into a marsh. She is using the fireman's carry, and carries him until she is at the point of collapse herself.

What I'm interested in is how this will impact the female character in days/weeks/months to come. Will it be something as minor as muscle soreness? Or would it be something more severe, like stress fractures, torn ligaments, other?

What would be a best case scenario? And average case? Worst case?

Mahalo!