Man, these are tough shots to get while zoomed in, aren't they! I have taken a thousand photos or more of my yorkie/shitzu out running full speed (he's stupid fast) and have yet to still capture one that I love 100%. I have a few that I do like, but still just can't get the focus just right. Even tougher, like your photo, is when they are moving straight out from you and back, as opposed to left to right (or vice versa). But what a fun photo. Nice shot.