Then

My first thought is to cram leetcode

I think you need to figure out what kind of job you're actually looking for. I'm assuming you mean "something not another startup"; if you're angling for another startup gig I'm not sure why you're concerned.

If you've been a C-level for 13 years, I'm sure not going to be interested in bringing you on as a programmer (unless maybe you have a notable project I'd know about), and probably not as a line manager. It's not just that you should be wildly overqualified, but as cajunboi said, you're not used to having a boss and I'm used to managers who understand what having a boss means. I spend enough time with HR without "my new hire still thinks he's the CEO".

I would de-emphasize the C-level job titles to the extent of changing them to 'Manager' or some such, and if brought up demure with something like "hey, it was a startup...everyone was C-something". Focus on the nuts-and-bolts of doing a job, so to speak. "I managed a team of 15 people and we did X" (citing 'on time' and/or 'under budget' metrics is good here), "I did X things to keep my team happy, productive and had low turnover", "I have a track record of taking a product from inception to X revenue", "I'm a recognized industry expert through my book(s) and participation in X events". Emphasize the team aspect of your accomplishments. Tailor the pitch to the job and don't just spam out generic resumes. And as always, work your network.

FWIW, I came from the startup side too, several times CTO. Now doing my penance in the corp world. :-)