If you are a short sprinter and you wanted to develop your speed endurance I think your reps should be <150m. I consider speed endurance to be the ability to sustain maximal, near maximal, or race pace velocity. In all of these instances, the 300m repeats will be run at a much slower speed than a race pace 100m or 200m. while they may be very beneficial I don't think they will be the most beneficial means of training speed endurance (at least by my definition of it) for a short sprinter because the speed of the runs will be a lot less than during race pace.
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