I have a son who recently turned 18 months old. He was premature, so he's a little behind on his physical development. He is walking, but not running yet. He's exploring his new found independence. He's not a big fan of sitting in a stroller for long walks (a favorite past time that has now turned very dramatic.

) While he can walk with me, the walks take a long time, and are never for more than a block or two... not a very good workout (at least for me!) I would like to teach him that physical activity should be an everyday thing.
Does anyone have any suggestions for a hearty workout I can do WITH my son? Or at this point in his development, should I do it on my own?
for reference, he weighs 25 pounds... A little too much to carry for long distances. That and he wiggles.