alohadreamer Stop being so hard on yourself. We ALL have runs like that.MORE THAN ONCE However they are the ones that make you mentally stronger. It could of been the heat, lack of hydration, the speed you were running, lack of proper fuel prior, etc etc. Don't sweat it. You already know that after completing one HM and having two 10 milers under your belt during this training your already ready to go. I would suggest on your next long run. Rest the day prior if possible. Be sure your  fuel supply is good (whatever you have been doing) Use a Hammer gel pack, GU, Singer etc 15 minutes prior to your start. Instead of worrying about your speed. Concerate on covering the mileage. Run slow and hold back. If your not telling yourself your going to slow. THAN YOUR GOING TO FAST. Break your run in thirds. Ok I'll run 3.5 miles than I'm walking for 20 seconds (just like walking thru a water stop) repeat as needed. Remember your long runs are building your slow twitch muscles. It doesn't matter how fast your going. It's time on your feet that you need. Try to stay around 70% max HR. You can use your last two weeks to work on your pace as your mileage decreases.

PS

In regards to feeling fat & old. Someone fitting that description passed me in Jan at the Disney HM at mile10 as I was on my way to a PR. Remember it's also a head game.

It is probably the heat. When it gets over 80 degrees things change. Run during the hottest time of the day and get used to it before it realy gets hot.

Not sure about how long you have been running, your gender or age, but I am a physical therapist with a teenage daughter who runs cross country and distance events in track and a few additional non-school events over the summer.  This began happening to her during her 3rd cross country season with difficult recovery after each race and struggling through practices and races to maintain her same times from the seaon prior, even though she had worked much harder over the summer.  This was worse during warmer weather.  She continued to get more discouraged until I came across an article on iron deficiency, which is a precursor to anemia.  This is not uncommon in endurance athletes, and especially females.  It might be worth having yours checked out if this continues.  They need to check your serum ferritin levels, not just hemoglobin levels.  My daughter told me if this had continued all season she would not have wanted to run anymore.  5 weeks after starting her course of iron supplementation she ran her 4K PR of 15:04.  Good luck.

Maybe it might not hurt to take a break. Let your body rest and recharge for a few days. See how you feel after taking 3-4 days off. It does wonders for me sometimes. Good luck in the HM!

Hi AlohaDreamer I don't know your running schedule or medical condition, and I don't want to sound alarmist or anything  BUT it sounds like your body is telling you something.  Its call heart disease.   I had exactly the same symptoms,  completely spent and fatugued, then I would recover. and then the same thing would happen again.  Please, please go to your doctor and have a complete cardio tread mill test done, along with a complete medical check up before your next marathon.  One in four has heart disease, even runners.  Many times ther are no symptoms at all.   I was lucky and am still running.   Daytona Runner