As I ponder'd in silence,
Returning upon my poems, considering, lingering long,
A Phantom arose before me with distrustful aspect,
Terrible in beauty, age, and power,
The genius of poets of old lands,
Walt aspired to be a famous poet. Not only that but he continuously revised his writings in his collection  Leaves of Grass . This poem itself could have been in the mix of poems he was looking over. 
 
 By  “linger” , he most likely means thoughts. He wonders how can keep improving his writings.
As I Ponderd in Silence