I am currently in week 5 of a 12 week schedule to train for my first half marathon. I had no problems in the first 4 weeks. I completed all the long runs (7 miles was the farthest at that point) with no problems. This past weekend the long run was 8 miles. Only 1 mile farther than my longest before. For the first time I was exhausted. It wasnt even like I made it the 6 or 7 miles I had easily covered before and then that extra mile got to me. I was going down hill after just two miles, just feeling tired and miserable. I pushed to the 4 mile marker and turned around to head back for the second half. I jogged in place breifly at the water fountain and then decided to walk a quarter mile. I felt like a failure. I hadnt had to walk during any runs so far and this was only about 4 1/2 miles into this one. My favorite runs so far had always been my 4 and 5 mile ones. Anyway I picked it back up for another mile but then started to feel really cold. I had goose bumps all up and down my arms and legs. I shouldnt be cold running in near 90 degree weather. Then I felt my arms and I had stopped sweating. Aside from being cold I wasnt feeling as bad as I did on the first 4 miles. Still feeling pretty cold in that kind of heat worried me so I walked that last 2 miles. So my questions are is it ok to walk during training? If I struggled with this run will I struggle even more in the 7 weeks to come? Is there some other factor that must have affected my run just on that particular day? I just dont understand how I could breeze through 6 and 7 miles and fail so miserablely at 8 miles.

I think you explained exactly what the problem was. Very likely the temperature and/or dewpoint were more than you were prepared for and since you're probably using perceived exertion, you can't tell that you were trying to hold a pace that you were not acclimated for in the particular weather conditions. Furthermore, dehydration may well have been an issue, but you would only know if you weighed yourself before and after. Each time you go for a run, take note of the temperature and dewpoint - these are important data points to indicate whether you will need to slow down or to explain a bad run. Also, if you lose more than 3% of your body weight due to dehydration, that will have a very significant effect on you and possibly shut you down.

I agree with what formationflier is telling you. 90 degree heat can be very dangerous in terms of heat exhaustion and dehydration, especially if the humitiy is high. You did the right thing by listening to your body. In the future, consider carrying fluids and running early in the morning before it gets so hot. (I start my long runs at 5:45 or 6 am usually because I never want to be out past 9am for a 20 miler.)

Also, I would use this run to judge future runs. Everyone had bad runs. I was amazed at how crappy I felt after the first time I did 12 (I didn't hydrate enough) but when I did 14 the next week, I felt great. I did 20 this morning without an issue but 18 was really tough.... sometimes there is just no rhyme or reason why a certain run will be so tough but in your case, I think heat and dehydration did the job.

Keep going and good luck with the half!