I am a oostgraduate in translation and I'm taking part in a literary translation contest for training translators. The translation test consists of a translation (into Italian) of a text taken from "The Pilgrim's Progress" by John Bunyan. This translation test includes part of the author's Apology for the book and there are a few lines which are a bit difficult to understand. If I can't grasp the exact meaning it will be difficult to provide a good translation. Will you please help me understand what the author means in the following lines? Please not that I am not interested in how it should be translated (that's my job!). I just need help to understand what it means. The lines I'm talking about are those in bold, but I'm also writing the previous lines so that you can have the context:
When at the first I took my pen in hand
Thus for to write, I did not understand
That I at all should make a little book
In such a mode; nay, I had undertook
To make another; which, when almost done,
Before I was aware, I this begun.
And thus it was: I, writing of the way
And race of saints, in this our gospel day,
Fell suddenly into an allegory
About their journey, and the way to glory,
In more than twenty things which I set down.
This done, I twenty more had in my crown;
And they again began to multiply,
Like sparks that from the coals of fire do fly.
Nay, then, thought I, if that you breed so fast,
I'll put you by yourselves, lest you at last
Should prove ad infinitum, and eat out
The book that I already am about.