I guess I'm one of the wierdos that finds running EASIER than biking.

However, I'm 28 and I've been a runner since I was 12 (competitive in high school and college), and a year-round soccer player from the ages of 5-15. So perhaps I just never knew what it was like NOT to run! I literally grew into my body while running.

I find the heartrate issue interesting as well. I can maintain a lactate-threshold HR pretty easily while running and for quite a while- but if I try to hit the same HR on my bike, my legs just can't take it. I probably have strength imbalances from being a runner that are exposed by biking.

So- I don't know about there being natural runners or not. Perhaps- you do get the occasional insanely gifted athlete- but for the most part, I think environment has a lot to do with it.

However, I do want to point out one great benefit that running provides that biking and swimming don't- while high-impact exercise can be hard on your joints, it also acts to strengthen and build bone mass. As long as you don't overload your legs with pounding (ie GRADUALLY increase length/time by no more than 10% a week), your body will eventually adjust. I had a MRI in college of my knee- and when the doctor saw the slides all he had to say was "you've got VERY dense bones!". I took it as a compliment.