My car failed the smog check test (not because of emissions but because of the Idle Control Motor)?

i changed the idle control motor, and i went today to the place and they told me to drive the car for 400 miles i don't actually know why but he explained it, when i got home i read online that rather than do that, there is like...some sorta trick with cruise control, so does anyone know what that is? or something else i could do?