My wife has just started riding with me, and while she's relatively fit by "normal" standards, I'm a stronger rider. A few thoughts:

1. Treat your rides together as your "recovery" ride. Hammer solo, cruise together. Will help you actually take it easy on your recovery ride, which I find difficult when I ride alone.

2. If you are on a street or trail that doesn't allow you to ride alongside each other, have her ride in front. It'll be easier for you to adjust to her pace than to expect her to communicate from behind when you are going too fast or too slow. Also will allow you to keep an eye on her (I sometimes have to remind my wife to slow down when she pushes too hard and her riding gets ragged).

3. Expanding on that last sentence, don't neglect to tell her frequently to ride at whatever pace she finds comfortable. My wife hammered a bit too hard on some early rides because she was embarrassed about going too slow. I told her, "I'd rather ride at a slow comfortable pace than stop and wait for 10 minutes while you gulp down water red-faced."