Well done on the weight loss - it's not easy.  Depends on what the "flab" is - it could either be lack of muscle tone, or a fat layer.  You may have met your weight target, but remember that square inch for square inch, fat is lighter than muscle.  

Firstly, carry on with the running and watching your diet.

 Then, do general muscle strengthening exercises for your entire body.  As you're running, squats (single leg) and lunges are a good basis, and I'd always recommend a good plank, working up to good quality press ups for the whole body.  A good press up works all your muscle groups - keeping your body absolutely straight.  Start with just one, and increase.  I was recommended to start off with press ups against the wall, then graduating to press ups with your arms above your feet - on a stair, or chair.  Finally, hands and feet on the floor.  At the same time, practice planks, and hold for as long as possible.  The important thing is always to keep your form absolutely perfect, and to stop as soon as it starts to go.  If you're not sure what I'm talking about, google "plank exercise" - there's plenty of illustrations on the web.

Good luck!