How is "Distance Walked" determined?
People were monitoring the progress of this stat since it was introduced in October 2013 but its mechanics are still being discussed in communities. The best tip I could find about its inner workings is this one :
There is some sort of speed / play limitation about what counts as "walking" so distance traveled while driving or biking doesn't seem to count. Additionally, some people report not having distance counted while running either. It may also require that there is some "activity" every so often to count toward distance walked. For now, the best bet on this badge seems to be to let it take care of itself while you work toward other goals like destroying enemy portals, linking and fielding, and so on.
I found an article here describing all the actions you can do in order to be logged by Ingress. Here they are :
- hacking
- deploying
- linking
- modding
- firing XMP
- recharging
- dropping / picking up
- checking missions
- NOT capsule loading / unloading
- NOT recycling
The author of the post also explains how distance is calculated based on these actions, and even show a picture to illustrate his explanations.
The speed limit is real, and is "claimed to be roughly 6 km/hour".
The Ingress Wikia Trekker article confirms the rough speed limit: about 7 km/hour. Another Wikia article, about Metrics in Ingress claims that the trekker speed limit is "less than 8 km/hour", and the speed lock "checks that an agent is going slower than 60 km/hour".
Beware though, the vast majority of what you can find around the Internets is only based on "user experience", since Niantic Labs never commented on how the trekker is calculated.