I had the chance to train around David for about a month or so. His training included hurdling 2x per week (anything from 2-10 hurdles), a lot of strength endurance drills some more specific to hurdling then others and special endurance work over 300-400m.
The one drill I remember seeing him do is with ankle weights and a hurdle against a fence. He would attack into the hurdle with his lead leg over and over again.
His training appears to be paying off.