How is this for a solution:
When you do your desk work, sit or stand. And go for a long walk every day (or twice). You don't need to do both at the same time. You don't have time to walk? Well, make time!
Your real walking will be better and more fun, and because of that, the sitting (or standing) work you do at your desk will be better (more productive, more creative).
This will especially be true if your job requires more creative and intellectual work, maybe less so if it is more 'mechanical' in nature.
Even in this Darwin was ahead of his time (or is it reverse, have we forgotten something?):
Rising early, he took a short walk before breakfasting alone at 7.45, and then at once set to work, “considering the 1,5 hours between 8.0 and 9.30 one of his best working times.” He then read his letters and listened to reading aloud, returning to work at about 10.30. At 12 or 12.15 “he considered his day’s work over,” and went for a walk, whether wet or fine. For a time he rode, but after accidents had occurred twice, was advised’ to give it up. After lunch he read the newspaper and wrote his letters or the MS. of his books. At about 3.0 he rested and smoked for an hour while being read to, often going to sleep. He then went for a short walk, and returning about 4.30, worked for an hour. After this he rested and smoked, and listened to reading until tea at 7.30, a meal which he came to prefer to late dinner. He then played two games of backgammon, read to himself, and listened to music and to reading aloud. He went to bed, generally very much tired, at 10.30, and was often much troubled by wakefulness and the activity of his thoughts. It is thus apparent that the number of hours devoted to work in each day was comparatively few. The immense amount he achieved was due to concentration during these hours, also to the unfailing and, because of his health, the necessary regularity of his life. (from http://www.darwin-literature.com/l_biography.html)
And of course, that other great and inspiring, but contemporary scholar NN Taleb knows this too (but he needed some help from Art De Vany):
After my Aha! flash, under guidance from Art de Vany, I embarked on an Extremistan barbell lifestyle: long, very long, slow meditative (or conversational) walks in a stimulating urban setting, but with occasional (and random) very short sprints [...] I saw serious changes in my own physique on every possible criterion [...] I also have a clearer, much more acute mind. (from http://www.fooledbyrandomness.com/whyIwalk.pdf)
Sitting per se is not bad. Sitting in the same position for a long time is bad. Same for standing. So vary between postures. And vary between postures and movement.
I hope this helps.