"Discipline is freedom" summarizes the attitude that if you have trained yourself to wake up early, stay on task, exercise regularly, etc., etc., then you now have the freedom to do a variety of things that you would not otherwise be able to do. By having the discipline to exercise, you now have the ability to freely use a more fit body, by waking up early, you have extra hours at your disposal, and so on.
To address your first question, I think Jocko would probably say: "If you form an intention to do something, and you don't do it, then you are mentally weak. The first thing to do is then to decide not to be mentally weak."
In abstruse lesswrongspeak, this would like something like: "It is most important to form a self-governing narrative of the form 'a mentally strong person would execute on their intentions regardless of transient impulses or mental resistance, and I commit with utmost resolution to being a mentally strong person'. Then you must continuously monitor your daily activities for adherence to this commitment and to this narrative-mentality."
Ironically, the Less Wrong deconstructionist approach of breaking the self up into multiple agents and carefully finding a minimum-enthalpy path through wantspace is itself antithetical to forming such a "simplistic" self-governing narrative, even if possessing and maintaining such a narrative were more effective.
So,
Make the decision and strongly commit to being a mentally strong person
Continuously monitor your actions to ensure they are the actions of a mentally strong person
Maintain this (for weeks/months/years?) until a new self-identity is formed.
If I've misrepresented something point it out, but this looks to me like a recipe for failure. It's missing fundamental parts of the human experience. People most often fail at their goals because of conflicting short and long term desires, forgetfulness and existing habits. Jocko doesn't adequately take that ...
I've recently been extracting extraordinary value from the Jocko Podcast.
Jocko Willink is a retired Navy SEAL commander, jiu-jitsu black belt, management consultant and, in my opinion, master rationalist. His podcast typically consists of detailed analysis of some book on military history or strategy followed by a hands-on Q&A session. Last week's episode (#38) was particularly good and if you want to just dive in, I would start there.
As a sales pitch, I'll briefly describe some of his recurring talking points:
The Q&A sessions, in which he answers questions asked by his fans on Twitter, tend to be very valuable. It's one thing to read the bullet points above, nod your head and say, "That sounds good." It's another to have Jocko walk through the tactical implementation of this ideas in a wide variety of daily situations, ranging from parenting difficulties to office misunderstandings.
For a taste of Jocko, maybe start with his appearance on the Tim Ferriss podcast or the Sam Harris podcast.