Puppy exercise, should they just do what they want?

I have a 5 month old English Cocker Spaniel named Quimby and when let outside he runs. He simply runs and runs. If I walk around he'll walk with me but when he gets the zoomies he just flies all over the place. Today I went for a ride on the ATV and he ran along side for a few miles. Made me really wonder if the joint issues are for larger breeds. What's too much? I don't want him to grow up a fat slob from not ever letting him exercise. I do give him a nice treat after we've been out playing. Today, have a home on the Texas coast, I walked him down the pier and threw him off the end. Wanted him to learn to find the bank incase he fell in. He did great and not only that he hopped up, jumped back on the pier, then took off and jumped back in and did it again. So now he loves the water. Thinking swimming should be a great thing for him right? Or is it more a tendon or overstress thing?