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.

http://breakingmuscle.com/brazilian-jiu-jitsu/it-s-all-in-th...

https://www.defrancostraining.com/joe-ds-qlimber-11q-flexibi...

https://www.youtube.com/watch?v=3B-3Khbht5s

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.