If I do cardio, why do I need to do resistance training to tone up?

You'll always need both because, without building up your cardio endurance you will be at risk for heart disease. Choosing not to do strength training can cause your muscle to atrophy (Atrophy is the partial or complete wasting away of a part of the body). Strength training improves bone density, balance and sports performance.  If you tired of the aerobic and strength exercise, try Plyometrics. Plyometrics is a type of exercise training intended to yield fast, powerful movements, and improve the functions of the nervous system, for the purpose of improving performance in sports. If the reason why you don’t want to do Aerobic exercise and Strength training is because you lack time, try full body exercises. Full body exercises will give you a complete workout in half the time and get your heart rate up. So get out there and do something but, get the okay from your doctor and or assessment by you fitness professional/personal trainer before starting a new system of exercise.