Since you are not trying to lose weight (in fact you could stand to gain a little since you may be underweight for your height?), you can certainly get away with doing less exercise than someone who is trying to lose weight/body fat. However, the amount of exercise required to maintain your current level of body fat and muscle tone will most likely vary from person to person. My personal opinion, do what you're doing since you're clearly enjoying the current routine and see if it's enough to maintain. If you continue to follow a healthy diet I think it could definitely be enough!