walking running distance accuracy way off

My problem is the opposite. The running/walking distances that Apple Health records are way above the actual distances.

I'd suspected this to be the case for a while, because I would go on a four hour walk for example; the first time that I'd left the house all day, and it would tell me that I'd walked 35km. Looking at a map I couldn't understand how it could be more than 20km. I'd always given it the benefit of the doubt though, because I often go running on a 400m running track, and by counting the laps that I do I know exactly how far I run, and for this at least, things seemed to be accurate.

Today however, I went on a walk using an app called 'myTracks', which plots your walking route on a map. Again, this was the first time that I'd left the house all day, so any distance already logged by Apple Health was negligible. Once I got home, myTracks had logged this walk at 18.8km, which is consistent with the distance you'd expect I'd walked by looking at a map. Apple Health had logged me as walking 37.76km, almost exactly double the distance. And it's not the first time that I've noticed these wildly high recordings.

Any ideas of why this is or how I can fix it? Is it a software or hardware issue?

this morning I woke up, dressed, grabbed my phone 5s and a bottle of water, went straight out the door, and walked almost exactly 2 miles (previously measured by driving the route in my car). when I got home, I checked the health app to see how many steps I had taken, and noticed that the "steps" and "walking and running distance" displays indicated that I had just walked 6,372 steps aka 3.5 miles (1.75 X the actual steps/mileage).

the only explanation i can offer for this discrepancy is that the iPhone was in a lower-thigh pocket of my cargo shorts, and may have registered a false step for every inertial bounce of my not-very-graceful stride.