Maybe because my GF is extremely athletic and incredibly fast?? She does marathons, triathlons and CrossFit events on a regular basis. Zero body fat.
I run with her but more interested in lifting.
Then I surmise that your cardio conditioning is subpar.
I’m a distance runner myself. If you’re lifting, that won’t help you run.
If your gf has “zero body fat”....she is not a girl