I herd no more then 20-30 min of cardio where you are really pushing yourself. For the last three weeks I have tried this as well as weight training 3 x a week. In my cardio workouts I do 30 second intervals of running or steep inclines with a face paced walk in between. Before I would exhaust myself and not want to do it anymore. This way is tiring but it's not the same kind of tiring, it is recommended by trainers....good luck to you :)
I always heard walking for 20 min, then jogging for 10, then walking 20 min, and repeat for an hour will help you. Take it slow, you'll start getting used to it in no time! Good luck!