When training for a marathon, a runner's body composition physiologically changes. 

With consistent exercise, the heart improves its efficiency to send oxygen to the muscles. 

Muscles adapt to using and storing energy while bone density increases to make bones stronger. 

On average, the process takes around 20 weeks of mental and physical dedication, stamina, and complete exhaustion, to complete race that's over in a day. 

When people cross the finish line, or complete an impossible task, they often report feeling empty.

 Anticipation and motivation become replaced with apathy and disappointment. 

Science will tell you it's due to a drop in dopamine levels. 

Or maybe it's because we're suddenly faced with the fear of not knowing what's next.