The Guardian has a feature on what it would take to run a two-hour marathon, including this account from Berlin in 2012 with Mutai and Kimetto killing it out front:
As Mutai led the charge, he dipped his head in trademark fashion and a hint of a snarl appeared at the corner of his mouth – a look he refers to as, “Now, business.” Within his body, there was a tumult of exertion. His heart pistoned at over 160 beats a minute. His lungs bellowed half a gallon with every breath. He took three strides a second. In his cells, the complex process powering his heart and lungs and legs – the burning and resynthesising of adenosine triphosphate – created three times as much heat as energy. And so he was hot, too hot. His body, losing around three litres of sweat an hour, grew slick with moisture. Lactic acid began to singe his muscles. Everything but his conscious mind screamed, “Stop!” And still he ran.
First thought: Only 160 bpm? That's my jogging heart rate! Secondly, they did a 14:18 split between kms. 30 and 35. It's just nuts.
Read the whole article to understand just how much more effort it would take in pushing them over the limit of 1:59:59. It's 3%. At that level of competition, it's a huge leap—but not impossible.