We know that Virgil, like Milton, spent many preliminary years in planning an epic poem, turning over the possibilities, practising his skill and techniques on other genres before undertaking the most ambitious of all the literary types. He tells us himself at the beginning of the sixth eclogue that when he wished to sing of kings and battles, Apollo warned him to keep to pastoral poetry; and in the proem to Geo. iii he promises to make a temple of song whose shrine shall be Caesar’s, and on the doors will be pictures of the conquering Romans and of their glorious ancestors.
He did not in the end sing of Alban kings, nor yet (directly) of the battles of Caesar: he selected instead a subject which offered wide scope within his chosen area of interest.