At least she likes to work out! Running is a great form of exercise IMO, but it's true... alot of running will take the size off your legs and prevent you from putting on much muscle.
Have you ever seen a bulky long-distance runner? Or one with big quads? Sprinting, on the other hand is great leg work....
Maybe you could encourage her to do sprints and track workouts, at least, if you are really concerned about it... but damn, if she doesn't want to hit the gym, at least you got a girl who likes to keep herself in shape! Be happy for that!
