You use "it" when you are talking /refering about something said or mentioned earlier .
I love eating Japanese food . But , I dont like it (= Japanese fod ) when I have too much of it ( the Japanese food) .
I promise it > I promise what( something) I just said.
For 2 days I should not run because the doctor said it> For 2 days , I should not run because the doctor said it ( "it" here refer to what the doctor told you : you sould not run)