I am a group fitness instructor and mom of 3. Sorry to tell you chica, you gotta do some sit ups if you want to lose the belly! But GOOD for you for getting started! Cardio from a treadmill will DEFINITELY help you to shed pounds, which will help slim your abdomen and lose inches from your belly. But for the tightening back of those muscles and loose skin, you must focus specifically on abdominal excercises. Also, running can be very hard on your low back. Stregthening your core will help prevent any back injuries. Just take it slow and be patient with your body and the results WILL come. You gotta stick with it though! Google something like "treadmill workouts". I used to teach a treadmill group fitness class and I got a lot of different routines that way. ONe day you may focus on hills, the next on sprints, etc. There will also be walking options.
For your abs, I would recommend a fantastic video. It's called "The Tracy Anderson Method: Post Pregnancy Workout". It's about 45 minutes long and over half of it is abs. She really focuses on pulling the pooch back in. You can get the video for about $30 on Amazon. WELL WORTH IT!! Take it slow, though. Some of the excercises are pretty advanced, so do the modifications she talks about so you don't injure your back. Gotta build the core strength first. Hope this helps and good luck!!