The effect I wanted was of motion blur – the background sweeping past while the runners and mostly sharp, with perhaps a little blur in their legs and arms. It’s simple enough to achieve. You just track the runners fast enough and use a slow enough shutter speed. I was shooting into the sun, which was pretty stupid, but the background was nicer in that direction. That made autofocus a little tricky at times and obviously put the subjects in a bit of shadow, but one was fixed with persistence and the other with post-processing. From hundreds of photos, the one I picked really popped out at me.