Give important work enough uninterrupted time to become difficult.

Work becomes genuinely difficult only after the easy surface layer is done, and if you leave before that point you never meet the real problem. Short sessions keep you in the comfortable part indefinitely, which feels productive and produces little. The difficulty is where the value is.

Give an important piece of work a block long enough that you hit the hard part and have to stay with it. Notice the urge to switch tasks right at that moment. That urge is the signal you are finally somewhere useful.