There's a fine line to walk between planning and improvising.
Plan too much and you'll get diminishing returns as well as the additional stress of adhering to an over-optimized schedule.
Don't plan enough and you float from task to task, improvising your next move and making progress in a wishy-washy way.
The right combination is different for everyone. And the bright side is that regardless of how much you plan, it's never set in stone.