Learning to sail is challenging and exhilarating it is also not the easiest thing to do.

This is a run-on sentence.

It's two sentences. The first sentence modifies the second, so, in that sense, is a misplaced modifier, but it is not a phrase within a sentence, it is a different sentence. A run-on sentence occurs when two or more independent clauses (also known as complete sentences) are connected improperly.