Back when I was obese, I was lucky to get maybe two workouts in at the gym each week. I wasn't able to use the treadmill faster than like 4.5 MPH and I was lucky to stay on for 20 to 30 minutes.
I was also sitting most of the day at a desk. I would barely get up and move or even stretch my legs. I remember my legs would be so stiff they wouldn't even want to move.