As a kid 8-13 I was just below a competitive gymnast. My younger brother 4-12 did compete around this same time. During our twice per week gymnastics lessons I probably never once did what I would now call hard work.
I did progress over the years but never to the level of competing with boys my age. However my level of strength and fitness compared to non gymnast boys my age was much higher.
Fast fwd 25 years and I have now officially blown hard work out of the water. For the first 2.5 years of the last three I was pretty heavy into training, mostly cardio and some full body, weight lifting workouts. Usually about 15 hrs each week in non peak periods
While my fitness greatly improved it hasnt gotten even close to the levels I had even a couple years after quiting gymnastics as a kid.
This theory brewing in my head is that a steady progression of work just up to but not crossing the level of "hard" could slowly but surely increase performance and strength and fitness. All the while never getting close to being in an "over trained" state.
Do those of you who actually know and understand how the body and exercise work together (unlike me who has practically zero real knowledge here) have any thoughts about building fitness by doing lots of "work" at what "feels" like an easy effort? Allowing that the actual work load of what feels easy will increase over time.