You learn from all relationships. If your relationships with others are successful, then you learn what ingredients made that relationship so strong. If your relationships are weak/dysfunctional, then you learn what traits caused the relationship to become unhealthy.
I think you learn from relationships of all kinds in general. Even the lessons that you have learned in your close friendships with others.......can be implemented in your marriage to make it stronger.
I have some friends who felt that they learned the true meaning of love only after they got married. They felt that their previous affairs were in fact just strong infatuations, which they wrongly believed had to do with love. I even know people who met, became friends, fell in love......got married....and have a successful marriage, MashaAllah, because they continue to do what made their relationship a success in the first place.
The interesting thing is that with time people evolve and therefore relationships evolve. Your past relationships taught you important lessons......but you also continue learning new things about your relationship with your spouse and improvise along the way as the marriage evolves.