Enchiridion · §29

ἐν ἑκάστῳ ἔργῳ σκόπει τά τε προηγούμενα αὐτοῦ καὶ τὰ ἀκόλουθα, καὶ οὕτως ἅπτου αὐτοῦ. εἰ δὲ μή, τὴν μὲν πρώτην ὁρμήσεις προθύμως, ὕστερον δ᾽ αἰσχρά τινα τῶν ἀκολούθων φανέντα ἀπολείψεις."θέλω νικᾶν ἐν Ὀλυμπίοις." κἀγὼ νὴ τοὺς θεούς· καλὸν γάρ ἐστιν. ἀλλὰ σκόπει τά τε προηγούμενα καὶ τὰ ἀκόλουθα, καὶ οὕτως ἅπτου τοῦ ἔργου. δεῖ σε εὐτακτεῖν, ἀναγκοτροφεῖν, ἀπέχεσθαι πεμμάτων, γυμνάζεσθαι πρὸς ἀνάγκην, ἐν ὥρᾳ, ἐν καύματι, μὴ πίνειν ψυχρόν.

en hekastō ergō skopei ta te proēgoumena autou kai ta akoloutha, kai houtōs haptou autou. ei de mē, tēn men prōtēn hormēseis prothymōs, husteron d' aischra tina tōn akolouthōn phanenta apoleipseis. 'thelō nikan en Olumpiois.' kagō nē tous theous; kalon gar estin. alla skopei ta te proēgoumena kai ta akoloutha, kai houtōs haptou tou ergou.

In every undertaking, consider what must come before it and what must follow from it, and only then engage with it. If you skip this step, you will start out enthusiastically and then abandon the work in disgrace when the later demands become clear. Take the person who says 'I want to win at the Olympic Games.' Fine — but have you thought through everything that comes with that goal: the strict diet, the relentless training in heat and cold, the discipline over every pleasure?

— Enchiridion 29

Source: Greek (Schenkl/Oldfather critical edition tradition) + Long 1877 English translation