Xenophon, Memorabilia 3.5
“Certainly it is necessary—since the city does not provide public expenses for war—not to overlook it privately, nor otherwise to care for yourself less. Know well that you be no worse off in any other struggle or action because you have put your body in better shape. For the body is useful in everything people do. In all functions of the body it makes a big difference that the body is as healthy as possible. Even in something you might think the body is of little use—thinking—who doesn’t know that great errors come from having a sick body?
Forgetfuness, loss of spirit, ill-temper and madness often impinge upon perception because of the weakness of the body so badly that all knowledge is expelled. But for those who are healthy in body it is a great protection and they suffer no suffer no such risk of suffering this kind of thing because of the weakness of their body. It is probably that for those who have a healthy condition they will have the opposite experience. And, certainly, won’t anyone with some sense endure anything for the opposite of these things that have been mentioned?”
…
Anyway, is it not shameful to grow old because of carelessness before seeing how beautiful and strong a person you might be thanks to your body? It is not possible to witness this for someone who doesn’t make an effort. For it is not willing to develop on its own.”
Οὔτοι χρὴ ὅτι ἡ πόλις οὐκ ἀσκεῖ δημοσίᾳ τὰ πρὸς τὸν πόλεμον, διὰ τοῦτο καὶ ἰδίᾳ ἀμελεῖν, ἀλλὰ μηδὲν ἧττον ἐπιμελεῖσθαι. εὖ γὰρ ἴσθι, ὅτι οὐδὲ ἐν ἄλλῳ οὐδενὶ ἀγῶνι οὐδὲ ἐν πράξει οὐδεμιᾷ μεῖον ἕξεις διὰ τὸ βέλτιον τὸ σῶμα παρεσκευάσθαι· πρὸς πάντα γάρ, ὅσα πράττουσιν ἄνθρωποι, χρήσιμον τὸ σῶμά ἐστιν· ἐν πάσαις δὲ ταῖς τοῦ σώματος χρείαις πολὺ διαφέρει ὡς βέλτιστα τὸ σῶμα ἔχειν· ἐπεὶ καὶ ἐν ᾧ δοκεῖς ἐλαχίστην σώματος χρείαν εἶναι, ἐν τῷ διανοεῖσθαι, τίς οὐκ οἶδεν, ὅτι καὶ ἐν τούτῳ πολλοὶ μεγάλα σφάλλονται διὰ τὸ μὴ ὑγιαίνειν τὸ σῶμα; καὶ λήθη δὲ καὶ ἀθυμία καὶ δυσκολία καὶ μανία πολλάκις πολλοῖς διὰ τὴν τοῦ σώματος καχεξίαν εἰς τὴν διάνοιαν ἐμπίπτουσιν οὕτως, ὥστε καὶ τὰς ἐπιστήμας ἐκβάλλειν. τοῖς δὲ τὰ σώματα εὖ ἔχουσι πολλὴ ἀσφάλεια καὶ οὐδεὶς κίνδυνος διά γε τὴν τοῦ σώματος καχεξίαν τοιοῦτόν τι παθεῖν, εἰκὸς δὲ μᾶλλον πρὸς τὰ ἐναντία τῶν διὰ τὴν καχεξίαν γιγνομένων τὴν εὐεξίαν χρήσιμον εἶναι. καίτοι τῶν γε τοῖς εἰρημένοις ἐναντίων ἕνεκα τί οὐκ ἄν τις νοῦν ἔχων ὑπομείνειεν;
8Αἰσχρὸν δὲ καὶ τὸ διὰ τὴν ἀμέλειαν γηρᾶναι, πρὶν ἰδεῖν ἑαυτὸν ποῖος ἂν κάλλιστος καὶ κράτιστος τῷ σώματι γένοιτο. ταῦτα δὲ οὐκ ἔστιν ἰδεῖν ἀμελοῦντα· οὐ γὰρ ἐθέλει αὐτόματα γίγνεσθαι.
Diogenes Laertius, 1.37.2
“When someone asked who is lucky, [Thales said] “whoever has a healthy body, a sophisticated mind, and teachable nature.”
τίς εὐδαίμων, “ὁ τὸ μὲν σῶμα ὑγιής, τὴν δὲ ψυχὴν εὔπορος, τὴν δὲ φύσιν εὐπαίδευτος.”
Juvenal, Satire 10.356
“We must beg for a healthy mind in a healthy body”
orandum est ut sit mens sana in corpore sano
Seneca, De Beata Vita 3
“The happy life is one that matches its own nature—it cannot be gained unless first the mind is sound and in consistent possession of its own sanity. Second, it needs to be brave and vigorous. Third, it endures most nobly, ready for events, solicitous of the body and all that matters to it but not anxiously. Finally, it must care for all the matters which make life good but without obsession for any particular one—to be one who enjoys but does not serve the gifts of fortune.”
Beata est ergo vita conveniens naturae suae, quae non aliter contingere potest, quam si primum sana mens est et in perpetua possessione sanitatis suae; deinde fortis ac vehemens, tunc pulcherrime patiens, apta temporibus, corporis sui pertinentiumque ad id curiosa non anxie, tum aliarum rerum quae vitam instruunt diligens sine admiratione cuiusquam, usura fortunae muneribus, non servitura.
It would have been even more important back then given how disease can wipe out cities.