My Zeus is now 1.5 years and we too took it easy on him when he was young and only allowed short walks of less than 2 miles. Now he's older and we don't hesitate to go four as long as the summertime temperatures permit (Texas).
But, honestly, he really doesn't enjoy the last perhaps quarter of the walk. He's just in endurance mode. So, I don't see much point in it.
We've always walked him on concrete as we really didn't have much choice.
If you're trying to exercise your dog try swimming. It's very rigorous and they sleep like babies when you get them home, fed and penned.