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.