I train Jujitsu and I also sit a a lot. One of the things that can make it a little harder to stay fit is muscle imbalances that can occur from constant sitting.
If your posture is out of balance it might be a little harder to keep an exercise routine if your body doesn't feel right.
I'm in really good shape but if I don't stay stretched out and limber all of that goes out the window. I would say a good start to staying fit is minimizing the determent that siting does on the body.