Endurance cardio as opposed to HIIT cardio has been proven to slenderize your legs. Intervals will work your muscles a little more and build them. So while you’re doing cardio go at a low-medium intensity for as long as you really can. Endurance should be for quite a while since you’re at a low intensity so you’re burning less calories so you should run.. or whatever your cardio is….. for a longer time than if you’re doing intervals. EX run walk run walk etc.