Doing just sit ups wont expose a 6-pack on its own. It will com down much more to your diet and overall body fat.
If you can pinch a handfull of fat from your belly I'd recommend spending the half hour on the treadmill. Doing just sit ups wont burn the fat off you stomach (you can't spot reduce fat) so you're more like ly to see a difference if you combine a god cardio workout with your sit-ups but even then I wouldn't recommend workout of just crunches and variations thereof.
Also your diet is important for stomach definition, bet there are people on here who can give far better diet advice than I.