This is another reason I created the post. Some of us old or older folk wish we could "be a kid again", but part of why we aren't like kids is that we stopped playing long ago. Most adults I know couldn't sprint (run) to their mailbox at any reasonable speed... and it's not because the human body isn't capable of doing that at most any age, it's that most people stopped trying. If you watch young kids, they run everywhere... and for no good reason. Granted, I doubt any of us has the energy of a child, but what law of life said we have to stop playing, running, and sprinting? As mentioned already, a HIIT workout (sprint intervals, in a cyclist's case) has considerable research showing its provides the health benefits of endurance exercise in much less time, not to mention that if one is ever chased by a bear, at least you'll have some chance of getting away.