"In each action that you undertake, consider what comes before and what follows after, and only then proceed to the action itself" (v. 1, p. 171).
In this chapter, Epictetus intends to convince people to give themselves wholeheartedly to what they intend to do. He wants us to think about things fully, before acting. It later becomes clear in the chapter, that he specifically speaks to those who want to pretend to be philosophers, and never really fully devote themselves to becoming a Stoic.
Athletes must consider all the pains, injuries, and hardships they must ensure if they are to reach the pinnacle of success. And they can only focus on one thing. You cannot become the best player in basketball and baseball. Some have tried, but none have succeeded. Those who wish to become Stoic sages must do likewise.
"For your own part likewise, you're sometimes an athlete, sometimes a gladiator, then a philosopher, then an orator, but nothing at all whole-heartedly; no, in the manner of an ape, you imitate everything that you see, and one thing after another is always catching your fancy, but it ceases to amuse you as soon as you grow accustomed to it. For you've never embarked on anything after due consideration, nor after having subjected it to proper examination and tested it out, but always at random and in a half-hearted fashion" (v. 6-7, p. 172).
He lays out the type of training a Stoic philosopher must endure in order to win the prize. This list is actually a really good list of Stoic disciplines that even us Moderns can attempt to become more Stoic.
"Do you suppose you can eat as you do, drink as you do, lose your temper as you do, and be as irritable as you are? You must stay up at night, toil away, overcome certain desires, become separated from those who are close to you, suffer scorn from a little slave, be laughed at by those whom you meet, and come off worse in everything, in power, in honour, in the courts" (v. 10-11, p. 172).
The prizes of endurance in these practices? Serenity. Freedom. Peace of mind.
Epictetus concludes, "you must be one man, either good or bad; you must devote your efforts either to your ruling centre or to external things" (v. 13, p. 173).