As long as you are in your Target Heart Rate zone, 30 minutes of swimming, treadmill, elliptical will ALL help with fitness and some weight loss ... BUT .... remember the # 1 rule of fat loss.
DIET is 70% of the answer, physical training and exercise only account for about 30% of fat loss.
Resistance training and cardio are important for conditioning and vascular fitness , but they are secondary for fat loss.
Too many people still suffer from the belief they can "burn away" fat with exercise.
YES it helps, but when you look at a chart of calorie expenditures per hour you quickly realize you CAN"T train hard enough or long enough to shred fat is your eating is horrible.
Michael Spitzer - Author - FITNESS at 40,50,60 and BEYOND