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Don't go from 0-1 hours a day to trying to force 8+. Build up slowly.
Take 10 minute breaks every hour, walk around, get a drink of water.
Vary what you are doing. There are high effort activities and low effort activities that you can switch between. Don't spend 8 hours a day learning new concepts. Spend most of the day going over concepts you already learned and a small part going over new stuff.
Switch between environments, props, vehicles, figures, animals. Switch between composition, value, color, perspective and other subject specific fundies.
Switch between quick, small thumbnail studies and long, large format, finished pieces.
Try to put everything into the context of weekly project based learning that emulates what you would do if you were taking commissions or working for a studio or whatever your end goal is.
Don't just sit down and spend 8 hours a day doing proko's gesture section or drawabox or krenz 2d copying pokemon. Have a goal, have variety and review what you are doing every week to see if it is smart and productive.