All of the above, and, personally, its the circles. wheels, cranks, chain running sort of eliptically through it all, and feet endlessly doing circles. There's a smoothness that is fulfilling. I ran for several years...always felt herky-jerky. But cycling flows along-- a little life unto itself with its own rules, customs, lore. I like being 30 miles out, eating my Clif bar and looking at the gray clouds between me and home. A little adventurous. Finally, on days where everything hurts and nothing works, it feels good to get home and just stop.......but, like childbirth, its all forgotten next time with the fulfillment and excitement of starting out again.