Kim, be reasonable when you begin your workouts. You can do more to discourage yourself if you jump in full throttle and over do it, waking up sore and tired the next day.
Start slowly, try walking for 15-30 minutes a day, take your son out on a walk. Or chase him around the playground or follow him when he rides his bike (do 4 year olds ride bikes yet or are they still on 3 wheelers?) . Make it something fun for the 2 of you to do together. Then you'll look forward to doing it everyday. K@T is right, you may not have a lot of energy in phase 1 for a lot of exercise.
I was walking to the bus stop and back home each day at first. It was a total of about 30 minutes all combined. I did that for several months, then I added a Pilates tape, started taking the stairs at work instead of the elevator, parking in the farthest parking space and walking fast to the store, getting off the bus one stop early and walking home farther.
Now, I run 3-4 times a week, walk 5 days (because I still take the bus) weight lift 1-2 x, pilates or some form of flexibility 1 x. The running I had to join a running club to do because I won't do it by myself, but in a club people expect to see you. I'm now an advanced beginner since I ran a 5K last month, don't get too excited-it took me 45 minutes to lug myself around 3 miles. But at least I'm doing it.
You will too eventually, as the weight comes off you'll have more energy to exercise which helps the weight come off even better and you'll feel so much better. So it's a wonderful circle.
Sarah