If marathon distance was 30k, marathon running would be easy. Running would be “fun”.
Every runner will tell you the hardest part of a 42K or 26 mile marathon is the last 12K or the last 6mile. At this point, your pace slows down, it is the point referred to as “Hitting the Wall.” or “Bonking”
No one knows where the word came from but at Boston marathon, the oldest marathon (1897), when the runners reach the 30k/20mile mark they are greeted by a hill, they fittingly call it “The Heartbreak Hill”
Some runners reach “the Wall” earlier, others later, it depends on your level of fitness. In my first marathon I ran comfortably up to the 30k mark, after that, my pace kept going downhill. In My second marathon, 4 months of marathon training left me better prepared. It was only after I crossed the 35K mark that I started feeling a decline in my pace and I knew I had hit the wall.
What Causes Hitting The Wall?
Energy Burn out
All the carbohydrates you eat is converted into glycogen and stored. During running, glycogen burns quickly to provide quick energy. Runners can store about 2,000 calories worth of glycogen in their bodies, enough for about 30 km or 18-20 miles of running. The glycogen is enough to get you to 30K mark before it runs out, after that, you are on your own.
If a motor engine runs out of gas/fuel it stops. The only way you can keep the vehicle moving is by asking someone to push it. In a marathon no one will push you but your body adopts to the situation. When glycogen runs low, the body must then burn stored fat for energy, which does not burn as easily as glycogen . At this point the only way to get to the finish line is by sheer will power.