I know we've beat this like a dead horse - but if you had two runners exactly the same Fast twitch / Slow Twitch- everything the same 17:00 min 5k - sub 1:20 1/2 . They both do the same speed and tempo workouts -
By increasing the long run and overall weekly mileage of one of the runners would he/ she run faster 5k / 1/2 marathon times? - say they're both running 50 miles a week and you bump one up to 75 MPW - the extra 25 miles all easy miles.