I've found that Pentax AF just isn't fast enough for capturing my dogs when they are coming at me as well.
Probably if I had a Chihuahua it would be different, but I have a Belgian and a Rottie, both of which are pretty fast animals when they get going.
I actually have better success by focusing where I know they will be and then tripping the shutter just before they get there. One has to develop a feel for this, as there is no formula, there is a lag from the time you decide to push the button to when the shutter opens, and this has to be accounted for.
Stopping down will help a lot, as will working at longer focal lengths with the dogs farther away, since the AF will keep up more easily.
Also, rather than running them right at you, try having them run somewhat off to the side, again, this will let the AF have a better chance of keeping up.
I'll also try to slow them down a bit by running them in shallow water. It makes my job easier, and the images are more dramatic.
A couple of examples to give you an idea of what I mean. Both of these were shot with a manual focus 80-200 zoom on the K20.