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.
More than a year and a half later, I get 10,000 steps every day and use the treadmill five days a week. I'm able to use the treadmill at 8 MPH and can do a 5K in less than 23 minutes. I also have a strength training routine 3 times a week. I still sit too long at the desk, but at least now I walk during lunch and get up every now and then. Before I lost the weight, I couldn't imagine getting as much activity in.
I'm motivated to exercise as I want to stay healthy. It's become a part of my routine, rather than something I dread doing. I'm also motivated as I want to lose the excess skin around my belly. I stick with the strength training in the hope that the belly will flatten eventually. It's hard as the results are much slower than the weight loss with the cardio. But it keeps me exercising so that's a good thing!
I know it's difficult to shift your mind into a mode where you want to exercise. You really need to push yourself to exercise when you first begin a program. It does becomes easier when you stick with it, especially when you lose weight!