Because it would be 16 miles of bliss and 4 miles of torture then. The point is the last section of the run is always significantly harder - it’s even the same for 5k
I've heard it claimed that an ultramarathon is fundamentally a different experience because while it definitely requires excellent physical stamina, it has a large mental component to it, as well as a much bigger focus on nutrition. Very different sort of race, I guess.
there are multiple cycles from highs to lows and back and then typically a larger dominant split similar what was discussed here for the marathon but scaled to the distance.
Yes. I've run numerous 50Ks, 50 milers, 100ks and 100 milers. I felt like crap after 20 miles in almost all of them. Most of getting better at ultramarathons is learning to keep going when feeling like crap. Oddly, the one race that was an exception is probably the hardest one of them I did on paper - in that case I was going so slowly from the beginning that I never really hit a 20 mile wall.
20 miles is still a challenge, and how many people run marathons because someone else is impressed if you run 26 miles, but couldn't care less if you run 20?