I'm having an interesting problem myself. The first few weeks, I had success with using the exercise app for running and walking. Having my phone in my at all workouts. I always used the "no goal" since I just wanted it to record my stats as I run a fixed loop in the neighborhood and walk a fixed loop at lunch around the buildings. Then 2 days ago something strange started happening. I'd exit the building at lunch, choose outdoor walk - no goal, and hit start. As I began to walk I checked my watch and noticed that after about 6 minutes, I had hit 22.78 miles! I had to cut my walk short of course, I was getting tired after such a long distance in such a short time.

I thought this a little odd. So I took these pictures.
Yesterday, I went out and did the same thing. This time I started walking and kept looking at my watch. 10ft, 20 ft, 50ft, 6.5 miles, 6.6 miles. this was after about a minute of walking. All the other stats seemed reasonable, but the mileage varies.
I stopped the tracking. I deleted it. I went to fixed program of 1.1 miles (which evidently was my longest walk it remembered). I turned that on and walked the mile or so, and it was fine.
This morning I went out for a run (first time in a week) and I started w/no goal. I ran to the end of my street and my watch buzzed and I looked down and I was at 55miles. Call me Flash. I stopped the watch, went to the fixed distance (that I know it is) and hit 2.8 miles as my goal. I ran the rest of my loop. I know the various points along the way that it used to say "buzz...1 mile...buzz 2 miles." and it did neither. I had run almost the 3/4 of my loop and it said I now was only at .48 miles and when I was done I had only done "50%" of my workout. So I ended up saving that. Mind you the activity app knows that I did something from about 6:30 to about 7:05 at a decent intensity, but the green ring only showed that I did a bit of my workout. sigh.
When I got back home I figured that something was amiss, so I reset erased and re-paired the watch to my iPhone. I restored from backup. Today about 2 I went to do my 1 mile lunch loop and the watched warned me that it needed 20 minutes of activity to calibrate. ok. I set it to the no-goal setting was still a mess after about 50 feet. 11 miles or something like that. I stopped and discarded it, and then I set it to a 1 mile fixed target. it seemed to work ok. unfortunately it hasn't synced that activity back to the iPhone yet (ya'd think it would be almost immediate). So I looked on my watch and it also says that I only did 3 minutes of exercise (although it does say that I walked 1.09 miles).
I like a lot of the other features of the watch...but I'm starting to miss my fitbit.