No way of determining when the line "This mornin', this evenin', so soon" was coined, but it has been added to a variety of rhymes, unrelated except that they state some action or state of being that goes with what has to be done as a result- and done 'This mornin', this evenin' so soon.
It may or may not have been belonged to a stanza at first, but it was easily combined.

How many biscuits can you eat
Went up town wid my hat in my han'
O Brer Rabbit you look mighty good this mornin'
I went up Zion Hill this morning
There's a man hangin' round (and the lezzie verses)
Ole mule stopped dead this mornin'

And dozens more

It has the utility of "When de good Lawd set you free."