I guess it's too late for this advice, but an elliptical trainer might have been a better choice for you. It is zero impact, so won't aggravate your knee condition, but will give you a cardio workout similar to a treadmill. Wonder what kind of guarantee you got with the treadmill (return it in 30 days if you don't like it?) If you are able to, swapping it for an elliptical might be worth a try.
current weight: 140.0