Not counting the weight, even average size runners may have knee pain. It can be caused by poor or incorrect shoes, mechanical alignment issues such as over-pronation (which will be compounded by extra weight), or even running too long on hard surfaces. If you're serious about getting into running, you may want to get checked by a podiatrist (preferably one who either is a runner or regularly works with runners). He/she will analyze your gait and can recommend the correct type of shoe (and possibly orthotics) for you. Until you get your weight down, you can do things to mitigate the pounding, such as running on grass, on treadmills, or even water running. Running fast also puts more stress on the legs. Follow up your runs with a lo-fat protein shake, ice packs on the knees (or an ice bath) to reduce inflammation, or even NSAIDs. You'll also want to do moderate stretching of the calfs, hamstrings and achilles.
Want to make a suggestion here. I am a runner with knee issues as well. But I have a different approach then many have here as I am a minimalist runner. I run in Vibram FiveFingers (also known as those weird toe shoes) and I have to say that I have no pain in my knees when I run. I used to have knee pain and shin spints pretty bad when I used to run in conventional running shoes and I tried many different brands/types. In fall of 2009 I bought my first pair of Vibrams and I haven't worn any other kind of shoes for running since. On June 1st I ran my first half marathon in my Vibrams. Now I know that minimal running using the forefoot/midfoot isn't a popular idea with many people but all of the talk her about poor shoes and over-pronation and orthotics just makes me cringe. So I just wanted to give you a different perspective. If you do decide to try them out, definately go slow and don't do too much too soon. Forefoot/midfoot running uses different muscles and your calves will be complaining at you at first until you get used to it. You could even try running completely barefoot on the grass or a track or another softer surface to try it out and see how your knees feel before you invest in any minimalist shoes.
I also agree with most everyone on the diet part. Eating right is an important part of weight loss. I did Weight Watchers a few years ago and quickly found out that I could not simply exercise away a bad diet. I would start first with portion control and small substitutions. Eat off of smaller plates which tricks your mind into thinking that the smaller portions are acutally more. Get a food scale and some measuring cups and measure out your portions with what the serving sizes are on the package. You may find that you are eating two or more servings without even realizing it. Substitute some of the things you nromally eat with healthier options - whole wheat bread and pasta instead of regular - things of that nature. You would be surprised that chipping away a little here and a little there can really add up.
Looks like you are on your way. I am in the same boat. Woke up one morning and said enough is enough. I am 49 also and need to get back into shape. Started the C25K. Have done three 5K's so far. My plan is to try and do at least one a month. I have also signed up for a 10K trail run in Aug. I think I need my head examined. LOL I like running the trails better than on the road. Keep up the good work and you will hit your goals before you know it.