結構（けっこう） is a な-Adjective that is actually used more frequently as a sentence altering adverb, than it is as a な-Adjective. What this means is that it is regularly used before an entire phrase (without な or だ), and it will modify the entire sentence. The most common meaning that 結構（けっこう） has is ‘quite’ (a bit/a lot).

• 結構（けっこう）走（はし）ったね。
  We ran quite a lot, right?
• 結構（けっこう）高（たか）いですよ。
  It’s quite expensive.

結構（けっこう） can also be used very naturally with の. In these instances, the meaning is similar to 沢山（たくさん）.

• 今日（きょう）は結構（けっこう）の量（りょう）釣（つ）れたね。
  We could catch quite a lot of fish today, couldn’t we?
Fun Fact
結構（けっこう） is used very similarly in Japanese to ‘no thank you’, or ‘I’m fine thanks’ in English. For example, if someone asks you if you want to try a sample in a shop, most people would just say 結構（けっこう）です.
• 袋（ふくろ）は結構（けっこう）です。
  I don't need a bag, thank you. (I’m fine without a bag)