Excess in exercise is a way to marry the protestant work ethic of a Midwesterner with the self-indulgence of a sybarite.
It is the ultimate in carnality, the mortification and the indulgence of the flesh in one, all released in one mighty rush of endorphins. Asceticism and hedonism are united.
Faced with the glories of the English Lake District, Wordsworth was moved to poetry. Faced with the Sierra Nevada, any northern Californian baby boomer worth his or her salt would strap on the Nikes. The knees may be going, the sciatic nerves may be jangling, but immortality resides in conquering just one more summit.
(Wordsworth did a lot of pretty strenuous walking, though, too--albeit I can't say that I can quite imagine him training for a triathlon. He would have been an earnest and well-equipped cyclist, that's my bet...)