I suspect your knees are telling you that you need to go to a running store to be fitted for different shoes. You don't have to be overweight to have your knees tell you this either. Anyone any size can experience pain from running in the wrong shoes for their gait. Or running in worn out shoes. That's my guess.

I was pretty much exactly the same size when I started! My speeds were even the same. I agree with the other posters about good shoes and stretching. I do yoga and that is helped so much. I'm starting Week 6 tomorrow. My speed isn't up much yet though (walk @ 3.4-3.6 & jog @ 4.0-4.2). I do Weight Watchers along with exercise for the weight. (5'3" 135lbs.) Enjoy the C25K.

The new shoes helped me alot, I bought some Nike "Structure" shoes and some insoles that the guy at footlocker recommended and today I jogged / ran about 2/3s of a 5k, I did stop to walk a couple of times with my running partner but for short stints. The best thing about it was that I didn't get the aches in my hip and my knees like I have been getting in the past few weeks. I probably could've gone another trip around my route but I was working out with other people and didn't want to hold them up. I highly suggest talking to a knowledgeable shoe sales person about how you run and the pains you are having and see if they have one of those analyzing machines to match you up with a great pair of shoes. I felt better today than I ever have after finishing a jog / run.