Clyde1820

I doubt it. Depending on whether it's training or merely exercise, and whether you're actually pushing the body to adapt, or merely doing work.

But I can't imagine 50mi of cycling a week would make much happen, even for someone "slamming it" each of those miles.

For someone otherwise fit, uninjured and with a body that still is noticeably quick and effective on healing, in my experience it's been this simple: the body keeps adapting until it doesn't need to, at which point you either pump up the volume (difficulty, challenge, variation, weight, speed, etc), or you plateau. The body's too efficient at adapting, but it'll only do it unless you keep pushing it.

Of course, fitness is a range of things, and not just strength and cardio. Agility, balance, recovery, power delivery, stamina, ...

Used to do ~100-120mi/wk, some years back. As basic transportation and general fitness when I was competitively running, back in the day. Never did get much better at "performance" cycling, though certain aspects (stamina, balance [ie, on crappy surfaces/trails]) improved. My focus was general cardio, general strength, and running speed; to that end, I did a range of running, hill climbs, cycling, swimming and rowing to tune the various physical aspects I wanted to improve. But I'm all but certain I would have had to push much harder in varied ways for much longer than 120mi/wk in order to have made serious gains in my cycling performance.

JMO