They'll get my keys and wallet, but they've got about two seconds after that to leave the scene before the hidden pistol finds it way off my waist and in their face.

Please be careful. A bloke with a knife needs less than a second to slice your stomach open. Doubt me? Give a friend a magic marker, and see how often you can draw and aim before you get marked. A gun is only a good defense against a knife if the knife wielder is about thirty feet away. Any closer than that, and he still has the potential to reach you and slice you to ribbons.