This is amazing - especially since I have no pockets and phones tend to work themselves out of pockets on long distance rides. This is one of the BEST endurance purchases I've made: Cashel Ankle Safe Kit
I've tried a lot of things, and when it comes down to it, you need to have your phone on you (not on the horse) in case of emergency. This doesn't interfere and can carry a lot of emergency items should something happen and especially if you get separated from your horse.
(However, I keep my GPS on the horse - if he makes it home without me, they'll know where to look for where he left me!)
Another one you can use is Google Earth. There is a "ruler" tool that lets you draw lines from point to point. Then it gives you the distances. It can be annoying if the trail is very windy to keep drawing the points, but straight distances are very easy. It is very accurate, however.