I think as long as he doesn't seem tired, he's fine.

Dogs will also build up tolerance like we do! They have so much energy and I think it's good for them to get it all out and get a work out like we do.
I take Franklin out for 30 minutes every day. I'm not sure how far I walk, though. When Franklin starts to slow down or lag, I take it as him getting tired and I pick him up.