2 mistakes...one is that your not giving yourself enough time. it takes a little longer for someone who is new to fitness to get into better shape as opposed to someone who maybe took a year off just to be lazy and came back into it. they can get their bodys back in a few months at most.
second mistake is that your not doing any strength training. cardio is good and all, but studies show that strength training for women actually burns more calories than just bodyweight and cardio. so by not doing legs or crunches or anything, those areas will only change a little. you can still do lunges to tighten up and lose weight. it'll probably help you out alot more.
the only way your going to get big muscles building up is if your using really heavy weights that you can only lift for 4-6 reps. but if you stick to high reps (around 15-20) with lighter weight...your going to be doing much better