During Horace’s lifetime (65 BC – 8 BC) inspiration was associated with wild bohemians who sought inspiration from god-like muses by avoiding washing and growing their hair and fingernails long.
Horace was among the poets who spoke out against inspiration. He, instead, urged poets to focus on their internal capacities. While he allowed that natural talent was helpful he famously wrote that "the source and fountainhead of writing well is wise thinking" (1.4.5) That is, the lyric poet could train himself in craft and in doing so, add to what was already present in nature by shaping it so that all that he saw and experienced was made beautiful and what he called “decorous” in the poem.
By decorous he meant that everything was distributed in the right way and given its proper importance. Metrics were strict during Horace’s time, and Horace was a master of them, so such distribution likely involved not just right thinking but also a careful attention to stresses and rhythms.
In short, craft was more important than touchy-feely inspirations. In that poets crafted what was there in the world, they didn't have any more problems with inspiration than, say, a construction worker does.