Can anyone tell me why 16 is so dratted hard? Why? Why?

But a necessary run if one aspires to breaking the tape (or staggering across the finish line) in a marathon. And I, good folk, more or less successfully completed my 16-mile run today. A little under 4 hours, which is not particularly scintillating, but I'll take it. I'm pondering whether to settle for 18 in two weeks (ironically, I just typed "eeks" instead of "weeks" - subconscious turmoil?) or to go for the Big Enchilada of 20, using the tried-and-true hell route of two laps around Longview. Decisions, decisions.