[Vivianne]

No amount of exercise is too much for a rotty as long as you keep one thing in mind. ALWAYS, ALWAYS let your dog dictate the pace. Watch him carefully. They are a strong breed and love to play, from my experience at any age. I would play with my female "Akasha" all of the time. I would watch for subtle hints that her heart was in it, but her body was getting tired.
We'd rest often between play sessions and if she was tired she'd rest quietly, if I was wrong and she wanted to play more she would become figity.
Watch him well during play time and use rest periods for observation.

A little common sense goes a long way.