What does she have that I don't?

Hi Everyone, It's not my first time posting here (had different names I can't even remember) but I still trying to find a way to be able to swim many laps without having to stop for a few minutes after a few. I have seen it all/read it all/tried it all to no avail. I have been suspecting that it has to do with my technique/breathing/muscle endurance/or all of these. I am a technical hermit and I can say that I have pretty good technique. Have been taking 1 on 1 coaching (I want to do that again by joining a swimming club that teaches recreational swimmers like myself). I've got many compliments as to my style. I can swim all strokes. When swimming freestyle I have high elbow, good DPS, etc. I have read tons of articles, watched days of tutorial videos, bought fins, pull buoy. I am fairly athletic, broad shoulders, lean and long (6'1") body I could go on and on for pages about my past with swimming and the things I have tried, but I won't. I wrote all these down so you get the picture a little bit of my background. The question: I have a coworker who is a girl, short (I'd say 5'5"), a bit overweight, but the firm kind and not full fat - but she is not athletic at all just by looking at her. Although she was a member of an elite Hungarian jump-rope team. She went all over the world (World Jump Rope where they won several golds,etc.), so she is/was good at it. So this girl started swimming just recently and she can swim 1000 meters without stopping (mainly breasts, but free/back too) whereas I swim for years now and can hardly swim 200 meters before I have to stop. Also after a little preparation she was able to run the Marathon last year. again, she is not athletic and no one would pick her out of a line to run the Marathon... Her resting pulse is like 40. What is it that she has and I don't? What do I have to work on? I kinda know it: endurance, cardiovascular strength, but I would like others to say it. Sorry for the words I used when talking about her, but I had to be descriptive. Thanks