Starting dog running?

Hi, i have a 4 year old intact male reg cattle dog. He is VERY fit and VERY VERY active. I do fly ball and agility with him. As well as working on the ranch i live on, so daily he works 2500+ head of cattle and 400+ sheep. I do utility level obedience training with him. I jog with him every morning, and in the after noon I bring him down to the lake where he goes swimming for 2-3 hours, playing fetch and Frisbee, as well as jumping off the dock. And 4 times a week we go to the local dog park and he plays with all the dogs there. I do go horseback riding daily and he follows along on the trail rides, or on the quad or dirt bike which ever i take. However, he still has energy even after all that. The past week on the way down to the lake, i have let him run beside the vehicle (off leash), normally about 25-30kms. And let him choose the speed. He loves it. He could do it for hours, however, i have only let him do it on and off for 1 and a half hours- two hours. He could still go for a while but at a slower pace 10-15 kms, or a steady jog/lope. I normally only do this when it gets cooler before dark. How can i tell when he is tired enough that i should make him stop? Each go around on the lake trails is about 15 mins, then its by the docking shore, where i let him play in the water and cool off and drink etc for about 15 mins then we go with the truck again. This dog will not stop, he loves working and being physically active. Rawhides are finished in 15mins-1 hour, even the great dane sized bones. I give him treats in a jar/toys but he really isnt food motivated (he doesnt even eat steak or other treats, never mind dog treats lol)