5km twice a week really isn't very much exercise at all I'm afraid. Try increasing to 10km (there is a 5km to 10km app) and do some tabata interval training (google 'tabata interval') on the days you aren't running. There is an app ('tabata timer') for helping you with tabatas,
Exercise alone isn't going to lose you weight - you do need to look at your diet, however hateful that might sound. Cutting down on refined starch (bread, pasta, rice, potatoes) is a good start. You can compensate by increasing non-starchy veg and protein - eggs, cheese and pulses are very good at making you feel full. Google 'insulin resistance' if you need more convincing.
Unfortunately you can't 'blast belly fat'. There is no way to specifically burn off fat in one particular body area. You can certainly reduce your overall fat body percentage by diet and exercise, and you can increase the size ('tone') of your abdominal muscles with the correct exercises. Make sure you do a full core workout so that your muscles are balanced, rather than just concentrating on abs - you need to do back, obliques etc at the same time.
Even better, do whole body exercises. If you have access to some weights (DH?) then learn how to do squats, deadlifts and cleans which are the best whole body exercises there are for burning energy and building muscle.
If you are thinking of joining a gym, find one that offers Body Combat, Body Pump and Spinning. Those 3 classes together cover all the bases for cardio, flexibility/agility and strength and I can guarantee that the weight will fall off you like melting snow :)