Based on the search results, the answer to "what a marathoner may do around mile 20" is "hit the wall". "Hitting the wall" is a term used to describe a state of extreme fatigue and exhaustion that can occur around mile 20 of a marathon, where the bodys glycogen stores become depleted and the runner experiences a sudden loss of energy. This can cause a significant slowdown in pace and make it difficult to finish the race. To prevent hitting the wall, marathoners typically need to properly fuel and hydrate themselves throughout the race and train their bodies to handle the physical demands of running 26.2 miles.