I can run fast, not very far fast!

So, today, I went out for a 5 mile run. I did mile 2 and 4 at my 5k pace, which is about a 7:10 minute mile. It felt fine. And that's the thing. I can run fast for about an hour or two. I did a 7 mile tempo run last week in about 56 minutes. But when I run long, my abilities really suffer. I did a long last Saturday of 16 miles. Took me 2 hours, 37 minutes, which is about a 9:51 m/m pace. How is it that I can run a half marathon in 1 hour, 52 minutes (8:37 pace), but I can't complete a run that is just a little further at a much slower pace without major slowdown? If I'm racing a half marathon, I can run it easily under two hours. If I run slow for two hours, say, 9:30 to 9:45 pace, I'm dead by mile 13 or 14. This doesn't make sense. By mile 13 or 14, I should be fresh, ready to handle the last six miles of a 20-miler. But at 13 miles last weekend, I stopped, got some water, had a cliff shot, and was dead. The last three miles were each over 10 minutes, according to my Garmin 305. During the White Rock Lake Marathon in Dallas last month, I had a horrible run, finishing in 5 hours, 20 minutes, even though I had had multiple training weeks upward of 40 miles including a 45- and 50-mile week. This time was worse than my first marathon in 2005, when my highest week was 40 miles once.
So, I'm trying some things. I've been taking blackstrap molasses, which is supposed to help with cramping and electrolyte imbalance. Its helped some. My waking heart rate is much lower than what it was beforehand. I probably need to run my long runs slower. More like 10 minutes per mile, and keeping my heart rate below 155 (77 percent of my max heart rate). In 2005, most of my long runs were 150 or lower. This past marathon training and last year, I did long runs trying to stay below 160 (80 percent max).
I've got a 20-miler scheduled Saturday. We'll see how that goes.