Beginner workout for a boxing wanna-be with weight issues

Good morning all! I know there are quite a few threads on here asking for assistance with beginner workout suggestions, but maybe someone will think this is a different sort of request. Background: After many many years of being severely overweight, I finally took control of myself and started working out. I started simple: doing 30 mins a day on a balance ball (with the assistance of a workout cd) and severely limited my food intake. I started going to my local community center as well - swimming laps whenever I could get time, and ended up dropping 90lbs in about 9 months. Now, due to situations changing in my life - I started gaining weight back and felt overall I didn't have any strength. So, I persued an interest of mine in joining a real boxing gym. I found a wonderful trainer who is a former pro boxer and kickboxer. I have been working out with him for almost a yr now. I have no plans to "fight" but the workouts are too amazing to ever give it up. Now - to my problem and question to the boards..... I need to increase my cardio. I do cardio with my trainer (I have 3 sessions a week with him for at least an hr), but I'm not dropping the fat that I've gained since my intial 90 lbs loss. I started back at swimming laps at the Y by my house - but I find that I'm just lacking in how to start and what to do. I never was on any swim teams in my life. I know HOW to swim - and can do the backstroke the best - with freestyle being about my only other option (cause I don't remember how to do the rest of the strokes mainly). I find myself completely winded within the first 200m - but somehow push myself to do 600m-800m (but this is at what I suspect is a lesiurely backstroke pace). I am needing to drop about another 40 lbs at least. Any thoughts/comments/questions are welcome. Sorry so long... :blah: