And should I fail, my courage brings its commendation:
greatness aimed at is enough.

[Quod si deficiant vires, audacia certe
Laus erit: in magnis et voluisse sat est.]

Propertius (50-16 BC) Roman elegiac poet [Sextus Propertius]

Elegies, Book 2, Elegy 10, l. 5ff (2.10.5-6) [tr. Holcombe (2009)]
    (Source)

Talking specifically about writing epics about mighty themes and accomplishments.

Because of the corruption of manuscripts of Propertius and speculating from internal references, some 19th Century scholars (e.g., Lachmann) argued for a different division of the elegies into books, thus the translations here wich instead label this as Book 3, Elegy 1.

(Source (Latin)). Other translations:

And if my powers of song should fail -- to dare were surely fame:
Enough that I have had the will; no higher praise I claim.
[tr. Cranstoun (1875), 3.1]

Courage in weighty works must power supply,
And praise be mine that I've the will to try.
[tr. Moore (1870), 3.1]

But if my powers fail me, at all events my courage will be meritorious: in great attempts even to have had the will is enough.
[tr. Gantillon (1884), 3.1]

And if my strength fail, at least my boldness will be a title of honour; in great enterprises, the very "I would" is enough.
[tr. Phillimore (1906)]

But should strength fail me, yet my daring shall win me fame: in mighty enterprises enough even to have willed success.
[tr. Butler (Loeb) (1912)]

If I have not the faculty, "The bare attempt would be praiseworthy."
"In things of similar magnitude
the mere will to act is sufficient."
[tr. Pound (1919), 5.1]

What though I fail? That I have dared aspire
should be enough. I shall have tried. What more?
[tr. Carrier (1963)]

But if my strength should fail, why then my temerity
Is sure of praise, for in matters of moment
The intention suffices.
[tr. Musker (1972)]

And should strength fail me, my daring at least will win me praise: in mighty projects even to have wished is enough.
[tr. Goold (Loeb) (1990)]

If strength be lacking, surely ambition will be praised;
The intention in great matters is enough.
[tr. Lee (1992)]

But if I lack the power, then surely my courage will be praised: it’s enough simply to have willed great things.
[tr. Kline (2001)]

What though strength fails? Boldness is certain to win praise. In mighty enterprises, it is enough to have had the determination.
[Bartlett]