I haven't done any runway for a very long time, but I'd assume the same trick still works: focus where they're going to be and shoot 'em when they get there. That is to say that under less-than-ideal lighting, manual focus (or autofocus at a preselected point, disbled during shooting) is probably your best bet.
It helps if you can catch the models at a pivot point; there's generally a bit of a freeze pose or two at a turning point (whether at the end of the runway or at the ends of aisles in a meander floor show), but if you are stuck in the middle of the runway somewhere, you can pick a couple of marks to shoot for. Count on bursts both to get the best chance of good focus and to catch something other than awkward postures (or bad draping, accessories flying about, etc.).