It's always a good idea to do at least 20 minutes of cardio. Ideally this should be done each day before you begin the weightlifting portion of your workout. If that is too much to squeeze into one session, try to get your cardio in during the morning or alternate lifting and cardio days.

Here's a cardio workout that not only elevates your heart rate, but that will also eliminate that midsection.

        
  Workout Archives:        
  Muscle Addict's 5-Day Workout        
  Aging Addict Workout        
  Good To Grow Workout        
  Thigh Die Workout        
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Muscle Addict Workouts
    GOT CARDIO? Be sure to include at least 20 minutes of cardio into each workout session.  
             
 
Heart Elevation - Gut Elimination
  
  When you perform this workout, set your mentality that this is firstly a cardio workout and secondly an abdominal workout. Each exercise should be performed in succession. For 20 minutes, the only rest involves moving from one exercise to the other. NO BREAKS in between exercises, that includes water breaks, and chatting with gym cutties!
  Go through the following midsection exercises. 2 minutes each exercise = approx. 100 repitions each. 20 minutes total.
 
1

Standing Torso Twists

    
 
2

Lying Leg Lifts
 
3

Lying Crunches
 
4

Lying Trunk Twists

    
 
5
   Hanging Leg Lifts    
 
6
   Cable/Machine Crunches    
 
7

Standing Straight Leg Twists