Ali,
Muscle loss typically is not caused by the fact that you don't take in alot of calories, it has to do with the lack of protein on low calorie "diets". Medifast is a muscle sparing program. If you look at the meal replacements, you will find that they are almost the same when it comes to protein and carbs. Your lean and green is mostly protein, with some carbs. The overall program is low-fat.
Given the combination of the vitamins, minerals, and carb-protein ratio...... its a great plan, even if it seems to be very low calorie.
Don't fret, if you have been exercising, just cut it in half. If you feel weak, have another replacement. Have the bar or a shake before exercising.
After three weeks, start to slowly up the exercise.
You will do great on this plan..... you already have a head start.
Hope this helps.