oh boy, I was wondering when this would happen if I think what is happening. The thing about artists and creators is not our desire to work and create as long as we want, it's the ability if we can. When you're working nonstop, it's still uses a whole lot of stamina and strength; which is a roulette wheel depending on your state of your body. It's not a joke among artists that they'll get carpal tunnel, it's a manner of when you get it. If not that, it's just a huge strain on your body, that's why a lot of mangaka's also take long forced hiatuses or drop their work, it's because their body literally can't take it anymore.
Miyama may have been able to work his usual schedule and pace in his own body, but he forgets he's in a different person's body, which may not be hardy as his.
[Thank you to the translators and all the hard work! Great work as always!]