Fitness is not concern. Smart decisions and reactions under pressure is; smart play means you maintain game momentum and tackle less- work on that!
I think fitness is a smaller part of it but you are correct. Watching Penrith as the standard - literally have the close-to-perfect game at the moment - their players are smart, aware and alert in each moment. They run with purpose, and watch the oncoming defence, thinking ahead rather than reacting at the last moment. They are not all necessary superstars but when the bulk of the team is playing with this mentality, tenacity and confidence they make each other more impactful players, and feed off each other. With this they make the bounce of the ball and 50/50 moments go their way - with the energy they bring and the masterful way their halfback keeps the ball on a string.
Us in comparison. Lots of one out stuff, not very supportive of or instinctive about one another or the opposition, and usually do something panicked, erratic, or just wait for someone else to do it. We have been like this for years. Tiny bits of spark or physicality but no real plan or concentration.