Get a cheap mountain bike off craigslist. Let some air out. Stuff your panniers with weights.
There...you'll get a workout and she'll keep up with you.
I try to avoid biking with some casual acquaintances for the same reason you have. But if I had something with a good friend, I wouldn't complain. I'm very picky about who I'm "friends" with, and they're more important than biking.