Keeping your workouts under one hour is a good idea; just make sure that you work intensely.
If you are not at all overweight, you could probably ease up on the cardio. Doing cardio 4-5 times per week is a lot and you should be careful not to do it too intensely as you could end up burning muscle, not fat. The fact that you have no fat to burn really means that cardio may be irrelevant, although it is important in terms of maintaining a healthy heart. I would suggest cutting out the cardio for now. If you are lifting intensely and *correctly*, you are giving yourself enough cardio anyway.
Going heavy with low reps is the way to build mass - just make sure that you are getting sronger every workout, even if it is only by one rep!