The idea didn't come to Arthur out of nowhere as it had been in the back of his mind even before he started playing DO. Because of his short stature and weak build, he struggled a lot and had contemplated trying to start exercising before.
But, the problem he immediately faced was time. Arthur didn't have any time to actually do that as his entire day was spent either working or studying or at school. By the time he finishes all of that and eats dinner, he is beyond exhausted.
He wasn't able to integrate exercise into his daily life so he simply forgot about it. However, now that his life had changed completely and his previous struggles were not a problem anymore to him, the idea of training came back again.
'Though, would training be useful considering that my body is getting stronger on its own?' He thought to himself.