I will be trying this but replacing the sit-ups with squats. Google Sit-ups and ask any real trainer and they will tell you that sit-ups are THE WORST workout. They do more harm on your back. Replace with squats which work your abs, and butt ;). Plus they burn calories quicker.
ab workout: 12 minutes long and consists of only three moves. Simple, but challenging. Set an interval timer for 12 rounds of 50 seconds of work and 10 seconds of rest. Go through the sequence three times.