Regular Physical Exercise is NOT Enough
This recent health research study that took a look at the U.S. National Health and Nutrition Examination Survey that involved 4,800 American men and women who wore a accelerometer throughout the day is somewhat hard to interpret, but seems to suggest that sitting for prolong periods of time is just plain bad for you, regardless if you engage in regular physical exercise.
Hence the wrong type of employment, such as sitting all day long in front of a computer, can adversely affect your health. If you find yourself in this type of a job, you should definitely try to take as many short breaks as possible. In short, periodically take short walks throughout the day.
In addition, taking a short walk during lunch is a good way to increase your daily activity levels.
Thus, whether you find yourself on a plane or at work, you always must periodically move around for better health. There was no indication that the physical activity had to be physically demanding, but you do need to move around a bit during the day.
time and cardio-metabolic biomarkers in US adults: NHANES 2003-06.
Healy GN, Matthews CE, Dunstan DW, ...
Eur Heart J. 2011 Mar;32(5):590-7. Epub 2011 Jan 11.