He's not going to appreciably improve his cardio at his age. Do you think that Conor, a professional fighter now for about a decade, hasn't been doing intense cardio training for a long time? It can get better, sure, but if it's not good now, it would be foolish to expect it to ever be particularly good, yet alone nearly good enough to keep the kind of workrate that would give him any kind of chance to have any meaningful success in a 12 round fight against Floyd Mayweather.
I'm not saying he's starting from square one, but he might as well be here. He's not facing some club fighter (and there are club fighters who would have very good chances of beating him in boxing) or even a journeyman (many journeymen would have very good chances of beating McGregor in boxing), he's facing Floyd Mayweather. Clinch fighting in MMA (or wrestling, muay Thai or any other number of disciplines) isn't infighting in boxing. There were people who talked about Chris Algieri, with his wrestling background, being able to nullify Errol Spence Jr. on the inside. Instead, he got completely bludgeoned there because Spence Jr. actually knew how to infight and Algieri didn't. Same goes for Kovalev against Ward (Kovalev with wrestling experience and training and a strength advantage still got outclassed on the inside against a guy who actually knew what he was doing there).
Floyd will smother and tie up his opponents sometimes to slow down the pace of fights and take away his opponent's initiative (the last two times we saw genuine infighting in any amount from Mayweather was against Maidana the first time out and against Cotto; since then he's not really bothered and instead has mostly held clinches on the inside as much as the ref will allow him to). What is McGregor going to do to prevent that (if it was particularly relevant to begin with, which it isn't)? Work to get his hands free and start working on the inside (aka learn how to infight)? McGregor is going to learn high level infighting in a few months (again, a skill that most elite boxers these days don't have)? Alright, man. If Mayweather were fighting an actually dangerous southpaw with strength and size advantages and infighting ability (like Errol Spence Jr.), he'd be in for real trouble, but he's not. He's facing a guy with no relevant boxing experience.