You are talking about creative work. Calculations are off for that one.

Unless you are a freelance artist that has a commission inbox overflowing you can't just be "more creative" for "longer hours" and earn more money.

The people you describe are addicts. And you should realize an important fact about addicts. They lie. Lots of it. To themselves and to others. They play the bad stuff down and fluff out the good stuff, if there is any. It is a justification mechanism.

Back to creativity. You risk burn out if you overdo and force it. Simple as that. Also health problems, due to stress. If you have fun doing the thing, you could put in some long hours at times. Just look at gamers playing some game for 20 hours straight. Or reading a book in one go. Doing an activity for long is not that unusual. But doing it for long for extended period of time, is something to be concerned about. Playing, lets say, an mmo for 16 hours a day for months is not healthy. It is an addiction. If it is work, that addiction is called being a workaholic. I guess similar effects like the runner's high are responsible.

Maybe start slow by doing regular activities that you deem non slacking. Like doing a morning walk.  Get your head clear, get some structure and routine, those should help with creativiy as well.

But I digress. Your question seems to be, if you should strive to be a workaholic. Since I consider it an addiction, you could just as well have asked, if it were a good idea to get addicted to drugs. And because we are talking about creative works, this is actually quite fitting, as there are lots of "artists" that brag about the creations they made, while high. Anyone that cares for you, should advise you against doing drugs or risking your health, so do not wonder that people tell you, it is a bad idea.

But becoming a workaholic is not the same as slacking off less. So do not use that as an excuse to read more manga ;-)

I think with creativity you need other things to wind down every once in a while. You mention reading and playing as bad things, but actually what they do is give you a break from the work you were doing. A break that you need because you are a human being and not a machine. You can teach yourself work ethic and get things done to a certain point even when not feeling it, but at the end of the day creative work comes mostly from your mind. You need to feel well to be able to make things. Last thing you should do is cut back from sleeping or hobbies, those are the things people start from on their way to a burn out.

Try to be more forgiving towards yourself. Everyone learns at their own phase, and there is room for all of us to make things and evolve. There are examples of people who have worked tirelessly to try and achieve fortune, but have fallen short. Than there are people who seemingly don't even try and get everything. Life is unpredictable and may feel at times unfair. But if you strive to create for yourself first and foremost, because it is what you want to do, than you will get something out of it no matter what. And it will show in your work too.

I wish you all the best on your creative journey.

Sounds like you have pent up emotions inside which needs to be expressed.

I could only suggest doing some exercises to do it. If that doesn't work, try seeking professional help.

There won't be a good strong answer for this but I can assure you that many of us have the same doubts as you (after all we are living in interesting times)
but in general:
Don't feel bad if you work hard
and Don't feel bad if you don't, rest is just as important (people who overwork are often not aware of it)
the sad truth is that many creative people only do it as a hobby and have a well paying (or not so well paying) boring regular day job on the side (those who can make a living out of their creative work are the exception)

most importantly be aware that hard work doesn't necessarily lead to success (there are many other factors out of your control)
but in case success happens: don't feel bad about it