I think they may have done it solely on BMI, as none of the other risk factors really applies to me (put it this way, I've only hit my 20s).
So it starts at a walking pace...how does it become a running pace at its maximum? Or does it stay relatively slow at its maximum (i.e., jogging pace). I did have a look at the Wikipedia article on the topic, but I'm just not quite sure what those speeds translate to in the real world.