Your extra cardio day if the 5 day split is what you choose
You can either do an interval run or do the 12-3-30 treadmill workout
When you incorporate a day of running/ some sort of cardio in this way: you strengthen your heart and build your endurance which in turn helps you lift heavier with WAY more ease. And not just that - adding in this one day will help you change the compositon of your body even more (if thats a goal of yours)
