In this special case it´s interchangeable to me. It would mean "a lot of something".
That is, let´s think of him thinking like "...thick wood spreading wide and black in front of me. A lot of it.
" and in Portuguese we can think basically that "era uma floresta densa, era sem fim (um sem fim de floresta)". https://dictionary.cambridge.org/dictio ... -something
This is my loose translation, a translation on the grounds of "sense" not a more literal one, so others may chime in and improve or offer other comments on that one.
As for the pair "end of it
" vs "end to i
t" in other contexts. We could think of the former as "fim disso/fim dele/fim da coisa a que se refere" and the latter as "fim disso/fim dele ou dela/fim da coisa a que se refere".
For example, it´s would be a different thing "o fim de uma dor" and "o fim para uma dor", in cognitive terms.