I was wondering, if i do more cardio like + 45 min more on my current 30 min, with proper diet and strenght exercises and calorie intake, could i preserve my muscle mass and just lose fat?
In short, Yes you can. But, as you qualified, you'll still want to do your strength training as it will help you burn the fat much more quickly along with continuing to build and maintain your strength.
The exact answer on whether you should do 45 min of cardio depends on your fitness goals. Do you just want burn fat? Do you want to build muscle? Or do you want to train to run a marathon?
If burning fat is important and endurance or long distance running isn't important to you then another possible alternative to cardio is HIIT. HIIT - High Intensity Interval Training, is the perfect method for losing fat while retaining muscle mass and it is great for the lungs and heart as well. It is also a good way to train for spontaneous activities especially if you also desire to burn calories.
HIIT is not a replacement for strength training and will not produce the same results in the short term or even long term. Strength training will burn more calories but will not necessarily do your lungs as good as HIIT or Cardio. HIIT also isn't a replacement for cardio if your goal is long distance endurance. Some people define HIIT differently, so I'll add this qualifier Weight training being -i.e. weighted squats, leg press, calf extensions, lunges, etc. and HIT being non-weighted High Intensity Workouts. Some HIIT techniques involve weighted exercise but they're applied with a different methodology. The focus with HIIT is to burn calories not to build muscle strength. One of the drawbacks of HIIT is that many of it's activities may put you at risk of injury. If you're concerned about injury or have had a recent injury, I wouldn't recommend HIIT.
More on why HIIT is not a replacement for strength training:
“EMG techniques make it possible to study recruitment order, the relationship between stimulation and the amount of force developed, the type of muscle contraction (concentric vs. eccentric) and the effects of fatigue. EMG analysis in my study showed the approximate percentage of the recruitment of muscle fiber types in the quadriceps of a trained athlete during execution of a one repetition squat with progressively increasing loads.
Starting with 60% of one-repetition maximum, the slow-twitch fibers contribute 60 percent to the effort; fast-twitch fatigue resistant fibers, 30 percent; and fast-twitch fatigable 10 percent. At 100 percent maximum effort, however, the percentage of slow-twitch fibers involved is only 5%, while fast-twitch fatigue resistant is 15 percent, and fast-twitch fatigable is 80 percent.
The implications for athletic-type strength training are clear. To develop strength in the fast-twitch fibers you have to train with heavy weights. Light weights contribute little to optimizing strength and power performance. “
For complete article see: http://www.drdarden.com/readTopic.do?id=412352
If your fitness goal is endurance, then stick with more cardio and less HIIT and strength. In either case, you'll still want to do some strength training. See these articles on why strength training is important for runners. 5 Essential Elements of An Endurance Training Program That Most Athletes Neglect.