some questions and thoughts

So yesterday. I got out of work, went to the gym. But on the way, I was falling asleep on the train - I promise, this is not actually an all about me post.
So I decided that I did not, in fact, want to run, even though I'm supposed to be training for a marathon and I have this thought alltogether much too much to be ok.
So I did the easy thing...I biked.
30 miles.
and then ran a measly two.
which brings me to the actual point of this post: what is it about the human body that allows certain people to over-develop specific muscle groups? Why is it that for so many a marathon is a [fairly] simple undertaking, where for others it is a feat similar to building the pyramids alone?
And why is it that I sad and read a book, relaxed, danced along while riding, and yet....those two stupid miles felt great? Whereas if I had just gotten on the treadmill in the first place......I would have felt very differently? Some pople spend their lives on a bike, and can't run a mile. The people I know that do marathons haven't ridden a bike since the age of 12.
Except for Kukla...which brings me to this: why is it that swimmers are good at everything, and made of Iron Steel?
*Research posts to follow.