I'm surprised that no one has offered this suggestion: talk to her. Tell her what is it about cycling that you'd like to share with her and why. Ask what her expectations and interests are. Is she competitive and an athlete? Or a recreational "smell the roses" kinda person?

I'm also surprised that no wag here (and I would expect it to be a "he") has answered your question by saying that he rides with both his SO and his g/f.

In any event, I'm envious of you folks who ride together. My SO is chronically ill and disabled, so I am unable to share the joys of cycling with her. Even a tandem is out of the question: not only could she not pedal, but if we fell, she'd be at serious risk of severe injury. It's particularly depressing on a ride--winter, summer, doesn't matter--through beautiful country and I see things I'd like to point out and share. I've thought about taking a camera along on rides, but that seems an inadequate, 2-dimensional solution to a 3-dimensional view.