I don't know your wife's temperament or her motivation for riding, so it's hard to give accurate advice. However, you should probably scale back the distance/time until she becomes more comfortable. You can gradually increase the distance/time each time you ride. Maybe just add a mile or two each time. If she's competitive or eager to ride faster, she'll probably let you know.

My wife is just getting back into cycling after about 16 yrs. Up until about a month ago, she couldn't do more than about 7 miles. Last weekend we did a 42 mile ride on the tandem. What really got her motivated was when I introduced her to some of the women in the bike club. A couple of them have small kid's and pull them around in trailers. My wife enjoyed their company and it really motivated her to start riding more seriously. She is now going out more on her own, beginning to get stronger, and really start enjoying the rides. At first, it was more pain than pleasure but her body is now beginning to become more fit.

I also bought her a Terry Butterfly saddle which she finds much more comfortable than the Brooks she used to have.

Hope maybe this helps a bit.

SteveE