The longer you run, the more your body will attempt to hold on to that belly fat.
I recommend that every other day you do short bursts of high-intensity running or other aerobics, and on the days in between do weight lifting exercises. Studies have shown that this technique is more effective in burning abdominal fat.
Also do some muscle building exercises which will increase your body metabolism and you can burn more fat.
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If you're first starting out then you should start slow, 10 - 15 minutes. and then work your way up. if you have been running and exerising for a whlie then you can go for miles and then back track would work. anything to get distance, a sweat, and increased heart rate. be sure though that you're not burning yourself out from the beginning, this will just cause muscle cramping, fatigue etc. drink lots of water and keep at it, nothing happens overnight.
To lose fat the most effective method is diet. A low carbohydrate diet (such as Tim ferris' slow carb diet) has been shown to reduce hunger so that may be a good option. once you lose a decent amount of fat you may want to try some crunches to add some tone.