I've worked out since way before medifast but still managed to gain weight. If I were just starting a program I would begin at 3x/week, that is the minimum. Don't try to overdo it. I go to a health club with classes, pools, cardio, etc and I love the eliptical machine, that is no impact cardio and it's great.
I have also been doing pilates (ria does them too
and Mike, you have been doing one of the pilates ab exercises for years and didn't even know it! No wonder you have a semi-sixpack.
One of the easier pilates exercises for the abs is just what Mike was doing, draw your abs in, imagine pulling everything up (if that makes sense) hold it, then relax. Another is picturing a clock on your abdomen, 12:00 is at the top of your abs over your belly button, 6:00 just above your pubic bone, 3:00 over the left hip, 9:00 over the right hip...now, going clockwise, contract 12:00 and hold for 10 seconds (don't forget to breathe!), release, then move on to 3:00, go around the clock a few times, then go counter clockwise. You can do these anywhere! In the checkout line, driving the car, waiting for your mf shake in the blender.
Well, try that, take care you all. The hard drive in my laptop died (one week after my warantee expired) so I will be on when I can.