Well, I agree working doesn't necessarily equal productive for some definitions of "working" and "productive". I would agree that its tough on a daily basis to sit down and smash out one task for 16 continuous hours. However most of my "work" involves the occasional scheduled phone conference call, unscheduled conferences (aka brainstorming meeting), fire fighting, administrivia, talking to people, etc. While my coworker smokes approx one pack per day I'll post on HN for a break, or I'll go on a walk around the block... Also I cannot code for 11 hours straight, but I can profitably intersperse unit and performance testing, design and planning work, not to mention working multiple projects more or less in parallel.
Its sort of like housework or yardwork. Could I dig irrigation ditches / swales in the garden all day every day? No, that's nuts. But doing a variety of productive "stuff" for a full day is no big deal at all.