Was it a casual walk or was it more like a forced march? I think Buddy could go nonstop all day so long as he could stop to smell the, uh...roses... every so often. When I keep him in a heel position and go at a quick pace for any distance he will start to get winded, and sometimes will try to tell me he has had enough.. you know, he will stop and look at me as if to say..."Hey, what's the rush!? Can we just stop for a minute? There's this pretty little beagle mix that left me a message over there by the sign post"
Seriously though, I have been told that extended repetitive motion without much variation, such as might be the case with a 3 mile jog, can cause joint problems in young dogs. 2 years is still young, so I would recommend including enough breaks and pace changes to keep it fun and interesting.