The cardio you do the day before your leg workouts will actually help you. Your legs will be flushed of any built-up lactate, so they'll feel primed and ready to work hard.
Although Bill suggests that you stick to the workout he's prescribed for you, if you come across an exercise that is painful or you dislike, you can swap it for one that you prefer. If you do this, though, remember that you want to have a balanced workout. So be sure to exchange a quadriceps exercise with another quadriceps exercise or a calf exercise with another calf exercise.