Hello and welcome.
Perhaps you are not feeding him enough food compared to all the exercise he is getting?
That's an awful lot of exercise in one day.
Have you considered feeding him more?
My 80 pound Lab was eating about 4 cups of food per day to maintain his weight and he was not getting that kind of exercise.
If his recent Vet check was good, I would assume he is not getting enough food or exercising too much.