There is an example sentence:
We found a secluded beach a few miles further on.
In this sentence, I referred to the dictionary and found that the part of speech of further is an adverb. However on is given both as a preposition and an adverb, with very many definitions.
In addition, this post suggests that we shouldn't rely on dictionaries too much for grammatical information anyway.
The answers and comments below give responses from trusted users which give opposing analyses—some indicating on is an adverb here and some saying it is a preposition.
So I can't figure out that what the part of speech of on is. Secondly what is the meaning of on in this sentence?