None of our rides are "training," so maybe this won't work for you. When I started riding my husband was at his fastest. He just rode at my pace or a little ahead of me. He did his fastest rides with other people, who he no longer rides with. I got faster just by going out and riding alone, getting faster and getting lighter bikes over the years. Now we are more evenly matched. If we are going out for a long ride, we stay together. Since most of our riding involves hills, we usually average around 14-15 on these days. Flat rides we average around 16-17. Now, when he rides to work (maybe 2 days a week) he'll average 17 even 18 and my average alone on a hilly "training" ride tends to be in the high 15's, including the 15% climb at the end. I will never be as strong as my husband, but there are times when I can climb faster than him. It's usually when he is pacing himself because he is tired. He descends like a rocket and that's my least favorite part of riding, so he waits.
We usually do our weekend rides with another couple. He is a bit stronger than my husband and sometimes takes off ahead, but they usually stay together. My friend is slow; hence I often end up in the middle, riding alone. At certain points in these rides, though, my husband stays with me and is perfectly happy. Our goal is to ride together until we keel over and drop dead....