The second point "20-20-20" is not only about your eyes, but even your spine, when you sit for a long time you are damaging your spine, you can notice you have

The second point "20-20-20" is not only about your eyes, but even your spine, when you sit for a long time you are damaging your spine, you can notice you have reached the limit when you are not sitting straight but you have your legs crossed or on a side, going for a 20 feet walk and then sitting again is enough to make you go back to the correct position. For the last point "walk", I personally love to walk, I used to go to work by bike but then I changed to just walking, my workplace is around 3 miles from where I live, so every day I walk for around 90 minutes (going back and forth), I noticed this helped me a lot, less back pain, more energy and also lost several kilos. If you are not lucky enough to work so close to your workplace or you don't want to walk so much, I recommend going out of the bus/metro a couple of stops earlier, and maybe increase the number of stops over time.