I'm 21 and I've been Primal for almost a year now, decided to sign up for the forums. I went primal when I discovered that I'm sensitive to gluten and probably lactose. I'm currently a exercise science college student studying to become a strength and conditioning coach and I already work as one for my D2 college for the Women's Basketball team.
I did cross country for my first two years of high school, got tired of people calling me skinny (5' 11" and 150lbs and 15% BF) and took up weightlifting instead (and mid distance track and field). My body stopped growing at about 175 lbs. After a year of eating primal i'm now 188 with around 9% BF and I recover better after workouts.
Right now I'm in the process of getting my long time girlfriend to convert to a primal diet (similar dietary issues as myself).
Man, I'm 31 now and I just WISH that I had started this when I was your age! (Instead, my diet was TERRIBLE, and coupled with a sedentary lifestyle and heavy smoking in my late-20's, I'm amazed I didn't just keel over.)
Congrats on the dedication and I believe that, just like with everything in life, the best way to lead (in your case, your girlfriend), is by example. She'll probably see your fitness improve and energy increase and she won't want to be left behind. Mark Sisson talks with Lew Rockwell about this very idea - taking care of oneself first, and then graduating to tending to others - here in this podcast interview.