I believe that distance running/walking engages various systems in our bodies that connect us directly to our roots. Primitive man had to walk and run for survival – thousands of miles a year. Through millions of years of evolution, the muscles, tendons, bones, energy systems and cardiovascular components adapted and expanded the capacity of the human body to cover long distances. Over this extended period, a series of psychological rewards also developed. This is why we feel good when we run and walk at the correct (conservative) pace for us.