When you're walking you apply about 1.2 times your body weight to the ground in Newtons (Fz = 1500N in your case). When you start running, this rapidly increases to two times or more. Furthermore, when you're walking you have bipedal phases, which means your body weight is carried by both legs. But jogging is characterized by going from bipedal to unipedal, so all the force is being applied to the one leg and thus one knee.
You're perfectly capable of walking, so what you are looking for is a speed at which the forces on your knees are higher than during walking, but not as high as during running.
What's also important is to reduce the moment arm your bodies center of mass has towards your knee. Because the muscles around your knee have to stabilize the joint, the further you put your foot (and thus knee) away from your center of mass, the larger forces they will exert on your knee.
Now the single most common injury in runners is a jumpers knee or patellofemoral pain syndrome. You can bet that the tendons connected to your patella won't like the strains when you start to run faster.
So we want two things: lower speeds and shorter steps. Luckily, with shorter steps and the same step frequency, you automagically get... lower speeds! Now how do we know we're actually doing this?
Well one great way is to listen to your body or in other words: talk to yourself, out loud! If you're able to talk normally, you're walking at the right speed. Because as soon as you start speeding up, the words... don't... come... out... steady... anymore... You get what I mean? I can't check it with a heart rate monitor, but I'd bet it would be around 60% of your maximal heart rate, which is ideally for burning fat.
If you're more serious about it, you can get a heart rate monitor yourself and an accelerometer (like a step counter only fancier) and keep track of yourself, but really nothing beats listening to your yourself.
