The important thing is you are exercising- Probably putting years of life back into your body.
I've been alternating between 4.2mph and 6.7mph at 1/4 intervals. Maybe that is what you can do to break that barrier.
Dieting is a big part of losing weight- but, you have to burn the intake of calories. When I quit smoking, gained 30lbs. I had to learn to burn calories when I exercise. I tried to burn all calories over 2000 taken in (So, if you eat 3000 calories of stuff, need to exercise off 1000 calories). Also, limit your fat grams to 65 per day (one cheeseburger, lol).
Hope that helps. It was hard work, but rewarding once you started the routine.
Music help keep me motivated for those hour+ workouts