Suzi,
The Polar F11 should also be compatible with your treadmill. In other words, just put on the the transmitter strap and watch and the F11 will read your treadmill. I have found my F11 is pretty accurate with my Fitness Center's treadmills.
Have you had a stress test done under doctor's supervision? I would do one in your situation.
I agree with Beverly. Work at your own pace. Each step forward is progress. We are not striving for perfection. Set small goals. Maybe use a pedometer to measure your steps taken in a given day. There are other means of cardio that do not require running. Using light weights with multiple reps can also improve your cardio.
Most of all, I am proud of you for trying. Your determination makes me work that much harder.
DO NOT GIVE UP! You can do this.
