What speed should i walk/run on my treadmill?

I am 26 years old and have not exercised regularly in 9 to 10 years. 2 years ago i gained about 50 pounds during pregnancy. I have not lost 1 pound and my current weight is 205 at 5'9". I got a treadmill this weekend and I want to run in the morning and walk for 1 hour at night. What speed should I use? I don't want to overdo it nor under do it but I know nothing about cardio. Thanks!