I have been a desk jockey for about the last 15-20 years. But only week days. Nights and weekends I was very active for the most part.
I'd love to blame sitting, and may be able to some...but probably overworked by back trying to do everything under the sun myself, and ended up with back surgery.
Still active, and still working, but still sitting most every day too.
The two things that keep me energized and with good focus are getting plenty of exercise and a sugar free-grain free diet.
I mean good exercise too, like jogging or cycling, basketball, etc... to get that heart rate going for a while.
The diet usually leads to the exercise more than the other way around I've found.
A walk down from the office to get a cup of coffee 4 times a day will never cut it.
Nor flying, sadly.
(waiting for the tired arms joke here)