Just give her some more time. Her time schedule for advancing in mileage is likely much different than yours. Also, be sure to have fun things on your ride.
This Friday, my wife, 67 yo and I rode together for 24 miles. But, we stopped for luinch at a nice sandwich shop in the middle of the ride, and we rode at HER pace, not mine. She likes to stop a lot and readjust things, etc.
This followed a 14 mile joint ride the day before.
When I want to go at my pace, I ride solo. I generally go about 30 miles at a much faster speed.
Isn't it great that in our middle to late 60's we can ride together?