I too can vouch for the changing power of cycling (and long distance running, which I discovered around the same time). It replaced a nicotine addiction for me, alleviated mood problems and continues to help me think clearly. I'm the sort of person who requires regular solitude--long runs or bike rides give me that (they can be deeply meditative) and force me to get some fresh air.

Arranging their life in such a way that regular commutes can be accomplished on a bicycle or by foot is one of the most valuable lifestyle changes anyone with a desk job can make.