My ex rescued/rehomed a GSP a few weeks ago. When he spoke to someone 'in the breed' and mentioned that he walks the dogs (he has an English Springer and a Large Munsterlander, too) for about 2 hours at a time they were horrified

They said that he was walking them for far too long

As he has had working dogs for about 20 years it threw him, and me ;)
HPR's, along with Spaniels, and some other gundogs, have to be able to work all day. If they can't work then they are no use as a working dog. I don't mean running all day without stopping, they have to rest along with their owners :D from time to time.
If you feel your dog will take more excercise than build it up gradually. After all, you couldn't run a marathon tomorrow, could you, if you haven't trained for it :)