Best workout on treadmill

Hey girls,

I’m trying to remain a similar weight I am now, but replace some of my body fat with muscle in order to tone up more and look leaner. I’ve been reading that in relation to cardio,long runs aren’t helpful for this sort of goal, and that HIIT is best.

I have a treadmill that only goes up to 12kph. Will this be fast enough? And if so any suggestions of a running regime?

I know muscular workouts will be required too, but I have already drawn up a plan regarding that.

Thanks in advance 😊