"Going For a Walk" vs "Exercise"

Why does walking down the road alone seem like exercise but pushing a wheelchair feels like merely taking a pleasant stroll? Thursday I took Maggie, my little patient, for a walk around the block in her wheelchair. The sun was shining warmly above us and a breeze was blowing. We just kept walking; we walked around that silly block three times! We watched a squirrel scampering in a pile of limbs and sticks left from the ice storm, admired a little boy's new bike and shiny helmet and enjoyed a patch of daffodils. We walked as long as I do when I do it for exercise but I didn't want to go in I was having such a nice time! Maggie doesn't talk but I'm sure she was loving it, too.

Why, I wonder, don't I enjoy my "exercise walks" this much? Maybe I need to get a wheelchair to push. Or a cute little dog to walk.