You have to walk at a faster pace to get exercise credit for walking. It doesn’t have anything to do with the length. My morning walk is 6.8 miles and I always, without exception, get exercise credit. My pace ranges from 12:45 to 14:30 a mile, depending on the weather and my mood, and that’s good enough to get exercise credit for walking.
That's quite a fast pace for walking - very few people are going to go out for a walk and consistently maintain 4+ mph for nearly 7 miles. That is quite fast. Is it really based on pace? I had always assumed it was based on level of effort/heart rate, but I'm not so sure. I've gone on some pretty intense "walks" on very steep inclines and gotten a high heart rate and then find out I achieved no exercise credit. One time I did a hike where I climbed nearly 1,500 feet in vertical elevation and got no credit at all.
My big issue is that the watch only tracks your path if you choose walk, run or cycle workouts. There is no 'hiking' mode. Or at least that is how it used to be, I haven't checked lately. I'd get credit if I chose run but then I'd mess up my average running pace.
Just a pet peeve of mine and some others that I know.