^ this is exactly it!  If you don't run and start it up, the beginning is no fun.  No one likes being out of breath and and the soreness in the legs and the discouragement of not even being able to run a mile (especially since new runners tend to run too fast).  It takes time for the body to adjust to the activity and longer for it to become "enjoyable".  No obeast is going to spend more than a couple of days doing it.  Especially 'cause they are going to eat more because "they deserve it" so will see no benefit for it.