I let puppies self-regulate any planned exercise. IE - at the dog park, I'll walk and throw toys every once and a while - not constantly, and watch for signs that the dog is getting tired.
When it's noticeable, it's enough and I stop and let them nap.
Just don't force anything and you will be fine.
Danielle, owner of...
Fraggle Rock Collars & ...
Fraggle Vom Richcreek CGC CA
Red Dog's Reconnaissance Man CGC CL1-R CL1-H CL1-F CL1-S USJ
General Patton Vom Winter Storm