Great question. I actually have two races that this hits home with. My 2nd half marathon was in Maui. We started at the Iao Needle and ran into Kihei (if you've never been there, the race started about 2,000 feet above sea level and finished at sea level). As an inexperienced runner, I figured going all downhill for the first three miles, I could just run as fast as I could. Lesson learned around mile 8. I was shooting for a sub 2 hour half. I ran the first 3 miles in 22 minutes (a little over a 7 minute mile). Around mile 8 I was walking and running the rest of the race. I did finish at 1:58, but I really learned my lesson that you cannot run full speed going downhill...just because it is downhill. I didn't beat myself up over it, I just took it as a lesson learned.
My second issue was last year's Pittsburgh Marathon. I was really shooting for a 3:40 minute marathon. I was on pace through the first 17 miles as I was running with a pace group and I was feeling great. I knew once I got to mile 20 or so, the mind games would begin, but I was still a few miles away from that point. Then all of a sudden, my legs started to cramp up and I had to walk. I kept trying to run and I could when I slowed my pace down, but that was only short-lived...as I had to continue walking. The last 9 miles absolutely were terrible for me as I would continue to walk until the cramps went away and then I would continue to run until the cramps began again. When I crossed the finish line at 4:07, I was so upset at the results. I beat myself up over it for weeks. So many people had difficulty with that race because all winter/spring we had trained in frigid cold weather and then on race day, the temps crept up to the 70's. It took quite awhile to get over that one, but then it finally hit me that there are so many people out there that also struggled with the heat, so many people out there that would kill for a 4:07 marathon, and that you really can't control what Mother Nature provides you. I've really adjusted my goals now and rather than running races with a set time goal, I am really running them for the enjoyment of the event. My goals will be to run half marathons under 2 hours and full marathons under 4 hours, but if I don't and I enjoy the experience, then I'm OK with that.