It's best to start short and easy and gradually make your sessions longer and more intense, even if you have to start with only walking 5 minutes at a time.
Can you do that 2 or 3 times a day? Try working up to 30 minutes a day by a minute or two every couple days. When you get there, try going the same distance at a faster pace. If you go too fast too soon you may get discouraged and quit.