The best exercise is one that you enjoy. Doesn't matter what it is, if you enjoy doing it, you'll be less apt to blowing it off, and more likely to see results.

The 'gut', (which I'm assuming you mean excess fat) or fat storage in ANY area, is merely a matter of calories in vs calories out. Situps, or any resistance exercise, will NOT effect your 'gut' if you don't have your diet under control. All you end up with is hard, muscular abdominal covered in a layer of fat. Running, or any cardiovascular activity, is an efficient way to increase calorie burning. So, in essence, running IS more effective at reducing bodyfat levels, in an indirect way.

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Because a hard man is good to find.