I have a mix of Border Collie with Australian Shepherd. Recently we moved to a country house and now he has a 24/7 access to big green fields and almost 24/7 human attention. Happy dog. But I'm worried about him going over his limits for his love of playing. Looks like even when tired he just continues to find stuff to do, run, catch, bark. Is that ok? Or should I put some limits? My other dogs are not that crazy and I never had that worry with them, even though they had the same benefits.
Border Collies and Australian Shepherds are both rather extreme working breeds, so a mix of both is an equally extreme high energy / high maintenance dog. Giving your dog the opportunity to burn off most of that energy on his own is actually a good thing. Don't forget to walk with him and do basic obedience training to keep a personal bond with him and keep him under control.
The only instance when I would worry is when the dog develops obsessive compulsive behavior. Common OCDs in dogs include hunting either shadows or speckles of light, constant barking, constant repetition of certain movements (like running in circles) and obsession with toys to the point that they ignore food and sleep as long as the toy is in sight.
OCD in dogs is almost always caused by boredom. If you suspect or notice that your dog has OCD, you have to train much more with him to stimulate his brain and distract him from his compulsive behavior.
I wouldn't worry too much. If the dog is able to stop or slow down whenever it feels like it, it's fine. When you read about overexercising it typically means small breeds or puppies are taken on too long walks and they'll just have to follow on leash no matter what.
Don't ignore the fact that you just moved and now he has this huge yard to protect. Give him some time to explore. And remember, just because you think he is going non-stop doesn't mean he is. He may very well have found a favorite spot on your land where he lays in the sun and naps before coming back home.
p.s. You might consider getting hime a "bear bell" so he can avoid (the less) friendly wildlife.