I think swimming may be difficult for you because you're not conditioned for it. Running a lot only makes you a better runner. When you try a different sport, you'll find that you're out of shape for that sport.
Take me for example. Last summer I ran 45 - 55 minutes per day, 6 days a week for 2 months. I thought that this would whip me into shape. But I was wrong, very, very wrong. In early September, I went to hockey try outs and I was the crappiest, most out of shape, weakest, coughing up a lung, ready to vomit, sorry excuse for a player. When I skated 1 length of the ice, I was done, I mean really done. All the drills with stops and starts killed me and I was stiff as a board after (I could hardly walk).
What I learned is that I lacked "Hockey Players Strength" that comes from playing lots of hockey. When you run a lot, you become efficient at running and in my case it actually made my game worse. It slowed my footwork down and my performance suffered. I was surprised that I made the team.
Running does keep your cardiovascular endurance up but you need to do more than run to be really fit. If you want to compete in a triathlon, you need to train for a triathlon. Meaning, you need to build up your swimming endurance as beginners do.
Overall to be really fit I think you need to do cardio (running), strength train, have flexibility and perform some type of skilled sport that pushes your limits.
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