Anecdotally, people who I've seen struggling with an 8 hour day tend to be less healthy (overweight, poor eating habits....). Whereas people who exercise more tend to have more energy.
Anecdotally, I've seen out-of-shape couch potatoes to be best prepared for the challenge of sitting all day and coding with junk food and soda. It's much more of a shock to the system of somebody with healthy habits.