We've recently come to this conclusion with our Cockapoo. His mother was a working Cocker Spaniel.

When the weather is poor we have often tried to get shorter walks in dry spells but augment it with as much ball time as possible to make sure he's getting enough exercise (since he generally dislikes bad weather).

It's become apparent that there's no possibility of satiety through chasing the ball though. He will simply go forever, however tired he looks.

I joked that as a Labrador will seemingly eat itself sick, a Spaniel will run itself lame.