Taking walks with my three year old could be considered interval training. Sometimes she'll run delightedly from one mailbox to another, looking for spiderwebs, while I power walk or even jog to keep up. Then she'll stop for several minutes to examine a leaf she's found. Run, walk, stop, walk, stop, run; it may not be cardio, but it's certainly fun!
One day a few weeks ago, she picked up a good sized rock, probably the size of her two hands. As she carried that rock, her pace was noticeably slower than it had ever been. She could no longer run to the next mailbox as she lugged that heavy weight. We both slowed down in our walk.
It reminded me of a verse in Hebrews:
Therefore, since we have so great a cloud of witnesses surrounding us, let us also lay aside every encumbrance and the sin which so easily entangles us, and let us run with endurance the race that is set before us (Hebrews 12:1).