> With no recovery days, there will be no gains.

That is true of any exercise regime with much intensity. For muscular activity: pushing towards anything like your limits technically causes lots of minor damage, which the body repairs back better. If you don't give yourself sufficient recovery time within your weekly routine you miss out on a lot of that benefit because the body's repair/improve systems don't have time to properly do their thing. This is one of the reasons¹ why overtraining injuries are a thing. In terms of cardio this still applies, the heart is a set of muscles. Mentally I think there is a similar effect, but pinning down a cause for this is much more hand-wavy and subjective compared to the far better understood² mechanisms of how the body repairs, regulates, and improves, physical structures.

Some people seem to manage with minimal recovery time, but they are either lucky³, kidding themselves, or storing up issues ready for a big nasty surprise later.

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[1] Other similar reasons include damage & fatigue in parts of the body other than muscles, and sometimes just being tired to the point of getting form wrong and hurting yourself through that.

[2] though still not entirely understood

[3] I'm counting being young as being lucky here. I'm trying to get back into running and other regular exercise (after a period of illness, looking after family with medical issues, and general burn-out) and the biggest thing getting in the way of improving from here is that I'm now in my mid/late 40s rather than early 30s like last time I was at this level of conditioning!