The only time I've ever run a level paced parkrun is when I've been on a go-slow and wearing a 25m pacing vest. Even then my breathing rate will up itself to 3 strides/breath from 4 at about 3k. I try to hold steady at 2 strides/breath for at least 2k and then hang on for grim death, starting this way is the best way not to start in O2 debt. After 1500-2k it's down to focus and leg speed, I try to imagine effortless turnover as if I were running on clouds in a dream.