easy, medium, HARD

We work out every day. Daily exercise tempers our bodies to withstand and sustain an average daily output that is 30% higher than rest days. This increase in output quickens our metabolism and sharpens our physical and mental capabilities. 

In order to maintain a daily discipline, there must be easy days. The easy days serve as recovery days. 

In order to achieve a progression is fitness, there must be hard days. We have to push it to get better.

Also, going faster makes us thinner. Ever seen a fat cheetah? Ever seen a thin Elephant? "IF YOU GET THE  FURNACE HOT ENOUGH, IT WILL BURN ANYTHING." Fast running pays for many sinful things.