I echo what @inuke has said. Articles on Hive down generate any income after 7 days. Maybe content writing outside high for any high quality requirement can pay close to 20 dollars per hour.
These days there are a few high skill based jobs available that can pay 30$ per hour. If that happens and if the income is regular, people can easily make 120$ in 4 hours a day. So even with a full time job as backup, it should be possible but as you said it has to be very hard work.
Basically be a machine for couple of years and convert that effort into money. But the trade off being health, peace of mind.
Doable but I am not sure if its a healthy approach.
My reference is my friend who is 34 but looks 60. He has the stock portfolio for him and he is ready for retirement financially and health wise.
Yeah sometimes we might have to compromise on the health for money. But if we can heal back and have a better standard of life at a later part, then why not take that effort when we have the health and when we are young enough.