something along the lines of theory/ logical thinking that would state how required it was to work out and develop discipline

The “develop discipline” part is pretty direct: there’s a lot of work to be done to get from here to communism, and it won’t get done absent a certain level of discipline. Developing some kind of discipline is simply a prerequisite to getting big things done.

Working out is much more tangential, but it can be a good way to develop that discipline. It’s also healthy for you on an individual level, which can only help you contribute more to whatever leftist project you envision.