The dog is learning more about pacing, and frankly I'm relearning it myself. I know this has been posted before here, but I think it's worth repeating. It's amazing how much we forget about training when it's been a while since we've trained. I mean, intellectually I knew that my calves might feel tight and that it would be hard to pace early in the run but it's another to experience it again. Luckily at this point in my running I recognize a lot about myself much faster than before and can correct sooner. This prevents injury and keeps the run enjoyable. At the end of the day that's my goal: enjoy the run and enjoy the benefits of being in shape.
Have you ever stopped running or working out for a while and then took it back up again? This should be fairly common considering the significant drop off in gym attendance after March :)