That's not a crash; just a minor fender bender
.
You now have something to build on and think about. Mile 21 is a well known wall for many and just knowing that can mess with your head at that point in the race. Or did you go out just a bit too fast? Or was it a nutrition issue? Was the weather a factor? I ask these same questions myself since I just get worn out weary about mile 21.
During last January's Disney Marathon I just started walking at about mile 21. I probably would have kept walking for the remainder of the race. Fortunately a half mile into my pity party, a pacer group started to pass me and I just joined the back of the group. They carried me the last 5 miles and I really didn't feel that physically beat at the end. I actually seemed to get a second wind. That tells me that I was prepared to quit mentally before I needed to slow down physically. Of course, you may be completely different, but don't underestimate what negative self talk and mental delirium can do to sabotage your race at mile 21.