I'm not sure how long you have been running prior to beginning your half marathon training, but if this is your first half and you are relatively new to running then I would say this can be expected and you just need to be mentally and physically prepared for it. I run at a HM pace that is still over 2:35 so it does drain my body of energy and I feel the same wall many others in the same pace groups I run in report in the 9-11 mile range.

Best advice for being prepared physically is to make sure you stay hydrated and replenish your body with some carbs along the way. Really, while there is a physical use for those carbs, it seems that general consensus is that the real benefit is mental. Those sugars derived from those carbs are to feed your brain more than muscle perhaps, but if you don't feed the brain it will make it almost impossible to convince yourself to continue. Then again there are also reports that blood sugar continues to feed muscle, so it could be physical. Frankly, I don't care HOW it works as long as I KNOW it works (and it has helped me whether mental or otherwise). I hear varying advice on timing but the advice that works for me is if you are running longer than 90 minutes (probably the case in your HM) you should take in your first carbs 45 minutes into the run, and then every 30-45 minutes afterwards. In my case, at a 2:40-ish pace, I take something at miles 4, 7 and 10. The amount of carbs varies on your size, but around 30-60 grams of carbs per hour (30 for the smallest folks, 60 for the largest). I like taking in something with added electrolytes as well, so something like Gatorade or GU does the trick for me (I like the GU chomps personally).

Then there is the mental aspect. You have to tough through it. Your brain is commanding your body to slow down because it is running out of glycogen to fuel the muscles and you are switching to primarily fueling muscles with fat (which burn slow). This is normal. You can keep going though. You have to make yourself do it.

Heck, my explanation is probably way too long. I'm sure someone more experienced can give a more concise answer (and maybe even better insights).

My best guess is that you were just not used to running in the heat. Bad runs happen to everyone, so try not to sweat it. With all of the race day excitement, it's unlikely to happen during a race if you are properly prepared. Make sure you are well rested and well hydrated prior to the start of the race. Do not over dress. If the race starts early and it is likely to warm up during the course of the race, I usually take along a long sleeve t shirt from my Good Will pile and ditch it on the course (some courses have a drop area, but I would not drop something I would not want to lose). Also, pace yourself appropriately. Be careful not to get caught up in the excitement and go out too fast. Take advantage of water stops if you need them. You will have a lot more support (aid and crowd) during the race than during your training run, so take advantage of it.

It's not a good feeling when your last long run heading into a race is a doozy. When that happens to me, I try to give myself another "goal" training run prior to the race just to make sure I can hit it and get some confidence back. This usually is not a long distance run, but maybe try to hit a certain pace or run a hilly route that I know will help with my mindset.

Trust in your training. You will do fine!!

When that happens to me I take Arthur Lydiard's advice. Listen to your body and when its done for the day stop.

Walk to the finish to get your time on feet if desired but pushing past exaustion in training just messes up your recovery cycle and affects the remainder of the training week.

I blame the heat. It takes 3 weeks for the body to acclimate to running in the heat. What you experience was textbook reaction to one of those early season runs in the heat. To combat that, you need to slow down right from the start when you know it's going to be hot. Maybe 20 to 30 seconds per miles slower than you normal easy run pace.

I agree with the heat being the culprit. We had a mini Spring heat wave last week and I could barely push out 3mi! I don't think there's anything you can do. For the first time in my life I was considering running on a treadmill...or getting some of those not-so-modest shorts. Thank God our normal weather returned over the weekend. Maybe the weather will shift back for you too.