Mangan advocates self-discipline and he does so while advocating a diet that is ultimately self-destructive. Now, I have noticed this sort of exhortation before; I'm sure the 9/11 terrorists were very self-disciplined. Self-discipline is a tool and one can use this tool for good or ill.
In fact, there is such a thing as "easy" discipline. Many people prefer a sort of militaristic discipline because it is actually easier than thinking. Just do it, and wherever possible, get a bunch of people to do it with you, the thinking goes. But what is it that we are doing? Sure, junk food is bad. But what about running? Some exercise is good for you, but how far do you go? The number of injuries goes up over time.
There are similar temptations in the Christian world. We can create a sense of comfort through repetition, and some forms of self-discipline can encourage a feeling of relevance, meaning, and glory. In other words, it feels good, but it isn't necessarily what God wants you to do.
If we find first the right course of action and then martial whatever self-discipline we have to acheive it, we are much more likely to acheive our goals, and we are also more likely to increase our overall level of self-discipline. I've lost 40+ pounds using a diet Mangan doesn't believe in; if I tried it his way I'd get increasingly hungry until I felt completely helpless. One way brings strengthens discipline, the other merely causes irrational and addictive behaviour.
Most of us are reasonably rational; if our actions acheive the desired result we will continue them, if not we will eventually change them. One form of change is to just give up, which is what tends to happen to people who use self-discipline in error, if they don't die or find something even more silly to believe in. In order to strengthen discipline, we need to see that we can accomplish something.
As always, I need to use a bit of self-discipline to end this post. There's always something more to be said, but the main point can get lost in a forest of sub-points.