Going on my own anecdotal experience, I think there is substantial marginal benefit to cardiovascular exercise even at the level of 4+ hours a day of said exercise.

For a while I've been doing 4 or 5 hours a day of cycling and it seems to have weird cognitive effects similar to those of caffeine. While exercising I seem to sometimes go into some sort of trance state where I'm pretty excited and can do lots of work (at the same time as the cycling) pretty quickly without it feeling like actual work. I can listen to the same dance track 30 times without becoming bored of it. I think this might be due to 'runner's high', or due to increased blood flow to the brain.