Hun, I would strongly advise against using shakes for meal replacements when you are trying to loose weight, they might work for some but honestly the second you change back to normal diet you put the weight back on.
If you are running as well you certainly dont need to stop your intake of food to increase your weight loss efforts, you just need to change your diet a little and intake more protein. I know that Grunty giggles has written a fantastic post about weight loss program and it is widly recognised structure she laid out there will work, the trick really is eating every 3 hours.
Im sorry I know people will disagree with me but I honestly dont agree with shakes as meal replacements, I dont believe for one second that they are a healthy choice.
Running increases your metabolic rate and you certainly dont want to hinder your body in its attempts to burn off food by not feeding it.
... Im sure others will come along in a second and disagree.
If you are interested in strengthening up your core etc rather then the sit-ups and endless workouts check out
http://www.bodyrock.tv as they do interval training with new work outs every day only lasting 12-15 minutes, you will see results a lot quicker following them then the countless hours in a gym.