"I go far away."
To expand on what others said, it is one of the few i-adjectives that can be used as a noun in their adverbial form (let's say く form): https://www.tofugu.com/japanese-grammar/i-adjective-ku-form-noun/ (most of those i-adjectives are meant to describe time or place)
とおく can be either a noun or an adverb. Both 遠くに行きます and 遠く行きます are correct in grammar, with a slightly different meaning: 遠くに行きます means to go to a faraway place. 遠く行きます means to go for a long distance. (Can't explain it quite well but the focus between the two are a bit different.)
Yeah looks like he changed the comment when I first replied :-) or just my misreading.
But I have explained the difference between とおく行きます and とおくに行きます where the former is going far and latter is going to a place which is far. Please refer to the long version in my earlier explanation.
Do adverbs like 遠く have to be placed next to the verbs they are modifying? The に between the adverb and the verb here threw me off a bit, but does this mean that I could say something like: 遠くに車で行きます "I go far away by car"? Will the adverb always modify the verb, regardless of placement?