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Forget the sit ups for now. Intervals and speed workouts are going to be key for losing the weight around your middle.
During a routine run, incorporate speed "spurts" I'll do this with songs that are playing on my mp3 player -- during the chorus or refrains I'll run hard, using the other parts of the song for recovery. If there's a bridge (in the song) I'll use that to run as fast as I can.
Another way to incorporate interval work would be a 10 minute warm-up, sprint for 1 minute, recover for 4 minutes, do this for 30 minutes of your workout, and include a 10 minute cool down. Increase the sprint time to 2 minutes and decrease the recoveries to 3 minutes as your body gets used running hard.
Variety is key -- vary your methods, times, even your workouts. You can do intervals during cycling, power walking, rowing, even with circuit training.
Save the sit ups for when you lose the fat around the middle.