Bicycle Touring & More. Other outdoor activity

Hello All - my name is Art and I am new to the site, I’ve been doing Triathlons for about 4 yrs and this year I am doing Escape from Alcatraz for the first time. I could use any advice from anyone who has done this race.
Thanks Art

I would someday like to race in a triathlon. I was hoping though that someone here could help me with a question I have always wondered about.

Swimming has always come easy for me. I also have no problem biking long distances. However running even a mile is very taxing on my body. I handle other cardio activities very well, but running always does a number on my heart and lungs.

My resting heart rate is 65 - I’m sure not great by standards here, but not bad. So I do not think cardio healht is the problem. I’ve also done a couple of various VO2 max tests and score well on those as well. So I don’t think it is my lungs. But it sure feels like both of these are the problem. Even running a mile at a 5 mph pace wipes me out.

My question is, am I doing something wrong? Why do other cardio activities come so easily and running is so hard? Am I just not doing it enough? I can go for periods without swimming or biking and get right back into it without the major stress on my body. What makes running different.