hey singlemom, you can go ahead and start exercising right away. Maybe start out with just 10 minutes a day and work yourself up. I am not a fan of too much cardio. I either take a slow 20-30 minute stroll, or I do HIIT (High Intensity Interval Training) for 20 minutes max. You will get a lot more out of Cardio if you push yourself faster and harder for 20 minutes then if you would go at a steady slow pace for 1 hour.
If you would like more info on the HIIT, just let me know and I explain to you how I am doing this.
Anyway, whichever way you decide to go, exercise right away is very important for your loss and maintaining when you get to your goal. 5 minutes of exercise is better than none.
Good luck.