I think your experience is one of conditioning or even breed type. I have German Wirehair Pointers which I keep in top athletic shape.

When we are in the field they will triple my distance travelled ( verified by GPS ). My outings are typically 8-12 miles and thus 24-36 miles for the dogs. Of course I need to keep them hydrated during this activity.

The behavior of running forward and looking back is most likely what we refer to as checking in. The dog is trying verify where you are heading/doing. In my dog's case they will range out to around 400 yards and then return to with 20 yards and run passed making eye contact as they run by.