Not lifting, not cardio...but more helpful than you'd think...if you can stick it for a month, if you even choose to begin, you'll notice the difference:
For those that are not so flexible:
Yoga may not be something most would consider but some of the poses stretch quite a number of muscle groups, and that's just one benefit.
Edit: I forgot to mention, the normal short duration is 12 reps on each side, so L then R and continue for 12. Or, alternatively some people set a time duration of 2 to 5 minutes, still alternating L and R.
The alternation is noted by the lead leg that goes back first to the lead up in to the push-up (or 8 points) posture.
Also, for those that are really inflexible, if you try this in the morning, you may get discouraged; give it a shot in the evening, when the body is warm, it really does make a huge difference.