No advice to offer, but some experience.
For me, relationships with married women in distant cities was complicated and frustrating, after the initial rush wore off. It's morally shaky ground, regardless of the status of their marriage. I found it to be a slippery slope that led to a breakdown in my moral character. In the most serious case, I have a lot of guilt over both our marriages breaking up and not being able to make it work once we were free to be in a relationship. The distance made it difficult, and neither of us were in a position to pull up stakes. I sought out of town relationships with married women several times over the years - it never worked out and was always messy - emotionally or otherwise.
I had to look at myself and figure out why I was repeatedly pursuing married/unavailable women in other states, and basing my attraction to them on characteristics that were a bad basis for a successful relationship. It took some intensive work to realize I wasn't being honest, and was avoiding intimacy by focusing on superficial aspects that didn't ultimately satisfy me. Selecting women far from my town kept me from getting too involved in many ways. I was selecting women that could be nothing more than a fling and trying to make it into a complete relationship.
I have since found a really great woman, who lives in my town, is emotionally and otherwise available, and is compatible across a wide range of interests, faith, child-raising beliefs and moral character. She may not be as hot or as young as some of my other partners, but the relationship is the best I've ever experienced. Once I got healthier, I found a healthier woman. In fact, we got engaged over Christmas. This will be the second marriage for both of us and we both have adult children.
If any of my experience is of any help to you, I'm glad I shared it!