There are still coaches out there, who believe, that GPS is a gadget and non of the elite runner are using it. Actually more and more long distance high end runners are getting it on and using as a hands on tool while running. Also coaches say, you should not rely on gadgets while running. What ? You should not rely on a power-meter while cycling ? Tell this to any bike coach, he will laugh at you very seriously. Heh ! For Track runners who has a coach with them, there is no need. Coach tells them the 200 splits and they can easily dial in the effort. But we do not prepare for half marathons and marathons on the track. I neither do for a 10km. Trail racing needs preparation on specific terrain, also we choose trail running to vary the terrain all the time, so no mile markers out in the mountains. Now it is possible to run intervals of varying distance, without worrying.