I got a new phone - now the iPhone 6s --- and I had trouble with the "Track your exercise". The displayed map showed deviations in the path I walked with lots of zig zags and the distance was way off. For example, I walked about 2.5 miles but the map showed 4.27 miles with all the zig zags and doubling back.
This didn't happen with my last iPhone. Do I need to set some GPS deiffeentkh on the new phone? I haven't updated to iOS 10 yet.