I'm nearing lesson 18(this one has 80 kanji, it's the first big one I've seen so far).

I have literally all day to do kanji. Would it be tremendously unwise to attempt to do this whole thing and the rest of lesson 17 today?

I think I could do it.

It is not unwise at all, provided you're confident in your retention abilities. The kanji in one chapter tend to have related primitives, so it can be easier to do them all in one go. In any case, there are bigger chapters coming, so now's a good time to try blitzing one and seeing if you can handle it!

I think the main risk is overwhelming yourself with too many reviews all at once in a few days.

Yeah man, up to you. You know your learning style better than anyone! Just keep in mind, the more you add daily, the bigger your daily reviews are gonna be for a while. I personally am doing 20 a day and while many days I want to do more, it's not wise for me long-term. I just did a 121 kanji review this morning and I don't think I can handle many more, so I hope that the reviews don't get much bigger!

It's like eating a big meal. It'll take a while to digest it, and a little more extra time in the gym to work it off... but in this case, it gets you closer to finishing.

Well, I did it. That makes 106 kanji today in total. I'll break 500 tomorrow. Awesomeness!

Not looking forward to tomorrow's review rates!

Yo playadom
I'm the living proof you can keep on even with horrible retention rates.
Dont be afraid, the worst it can happen is to get a huge failed pile.