Speaking from my recent experience.... for me, miles 16-19 were the most difficult mentally. It really felt that after 15, everything seemed to slow down. When I saw the mile marker for 20, it seemed the end was near since the mile markers now started with 2 rather than 1. After seeing 20, I thought, "huh" I only have a 10K left." That was my experience. Your mileage may vary 
dianna

Originally Posted by
katluvr
Did my 18 miles yesterday. I now know why a 1/2 marathon is my race. 18 miles is long and that really means 8 more to complete a marathon. I felt like I had a few more miles in me, but not 8. My overall avg pace is JUST within my goal (finish in 5 hours). So I'm a bit nervous that I won't make my goal time. But I felt pretty comfortable and I don't dread 20 miles (1 week from tomorrow) as I really felt like I could do 20.
I have a few minor blisters on a few toes, but nothing that will prevent me from running again soon!
K
2013 Specialized Myka FSR Comp
2013 Specialized Ruby Sport (carbon)
2014 Salsa Vaya 3 (steel)
2014 Felt Z75