SLOW-TWITCH FIBER: These fibers derive maximum benefit from low-intensity distance runs. But "distance" refers to duration, not mileage. Slowing down a brisk 60-minute run to a more leisurely 70 minutes increases the training benefit by 10 minutes! Kenyans are famous for running 8:00 mile pace on distance runs. Masters runners are notorious for going much faster.
Build Your Running Body | Running Times Magazine