"Hun er ikke klar til at løbe en maraton endnu."
Translation:She is not ready to run a marathon yet.
why is the Position of endnu in the end of the sentence? I thought that Adverbs Need to go after the full verb form "Hun er ikke klar til at løbe endnu en maraton."?
Well, endnu could go after the full verb, but the full verb here is not løbe. :)
At løbe in this sentence is part of an expansion of the main clause. The full (and conjugated) verb here is er, and that's also where you find the other adverb in this sentence, ikke. So you may say "Hun er endnu ikke klar..." But, just like the English "yet", endnu can be placed at the end of the sentence as well.