Master's sprinting and distance running are two decidedly different kettles of fish. Since I turned 40 I've had no problem maintaining, even increasing, mileage, but my raw speed has declined noticeably. I can't run 1K anything like as fast as I could when I was 20 but I can sustain a decent pace for 20k more easily, and the drop off isn't very apparent at that distance. I think the changes are neurological more than fitness; my eyesight and hearing are declining too, and my fine motor coordination is not what it used to be either.