I don't think I ever worked harder than training at age ~15-17. Like 25-28 hours a week training plus a couple of games. Always having at best slight aches, tiredness etc, and at worst hiding in the toilets before training with head in hands trying to eke out an extra five minutes because my body was screaming at me before I even got in the pool. And I did not have the work ethic to get past the level of a jr. national team "squad" member, not even a guy who played, just on the ~25-30 man squad for camps and in selection pool. I trained with/played with 5 of the guys who went to the Olympics in 2012, ~4 years after I pretty much dropped off, and all of them just had a different gear, unreal dedication (and we lost every game still). I mean, the physical challenge ofc is hard, but for me the mental aspect was harder. Constantly motivating yourself to do things you "have" to do that hurt/are physically unpleasant. I mean, I LOVED playing, but training could be complete hell.
For me working - now - ~40 hours a week requires less effort and is not as taxing, same goes for when I first left Uni and was working 45 hours with a 2 hour commute every day in a job I hated. Getting a Law degree and then a Masters degree was also easier.
Now I would counter that to a degree by saying that 28 year old me is way more equipped for the mental challenges that playing sport to a high level throws at you than I was at 15-17 (I could certainly train harder now than back then and "enjoy" it, I know my body better for one, and mentally being this age is waaaay easier than a teenager due to a fully developed brain, hormone differences etc!)... but that is the same for everyone, and the reason so many athletes drop out in their teens... but the ones who stay are generally, 90% of the time or so, guys with simply immense work ethics and mental strength, who as a teenager can push themselves to a degree that most cant. (talent of helps too!)
It prepared me really well for the "real" world though (though I am also pretty lucky to do a job I enjoy!). Discipline, self-motivation, self-respect, work-ethic etc.