As far as speed and running vs. jogging I think you should consider if you are using a treadmill that not all treadmills are created equal. I never realized this until I went from the one I had at home to one at my Dad's house. 3.5 MPH on mine felt like 3.0 MPH (or less) on his.
Weird...Anyway I think a good way to determine a speed that works for you whether using a treadmill or jogging/running outside is to incorporate a heart rate monitor. That way you can determine what speed is challenging but not too difficult for your personal fitness level.
Btw, I was waiting to start my C25K until an injury I had healed. I feel like I am almost there and can't wait to start!! I think I will give it a go on Sunday.