Four by four is the scientifically proven, efficient way to get in shape quickly. It works both for athletes and couch potatoes.
You alternate between running quickly for four minutes and slowly for three minutes. You do four repetitions, for a total of 28 minutes.
 Aerobic high-intensity intervals improve VO2max more than moderate training, Helgerud et. al., 2007
With Family Sharing set up, up to six family members can use this app.