Since I was the gentleman who commented on taking in energy during long runs, I feel the need to respond. There is not an iota of scientific reasoning that supports depriving oneself of carbohydrate intake during training---at worst, there might not be a benefit. I used a poor example that allowed Snell and Dave to side track the discussion.
First, I never stated that you could never finish 2 hour run. My comments were that reducing the training intensity or shortening the training time and thus reducing the training stimulus is not a great idea.
The idea to deplete type I fibers of muscle glycogen to get to type II to make them more oxidative is a common thought. However, you can still do this even when taking in CHO during the run. If taking in CHO allows you to keep the pace for an additional 10 minutes then you are training the type I fibers for an additional 10 minutes and still recruiting those fast twitch over the last segment of the run.
If one runs into a situation where you are energy low and pace slows or time at pace is shorter, then the training stimulus is reduced. Why would you want to reduce training stimulus?
Also, this assumes that type II fibers are not trained at any other time. This is also silly. Tempo runs and interval training recruit type II fibers.
I asked several folks at ACSM in Denver if they could think of a rationale for this practice; these included Bob Murray, Louise Burke, John Hawley, among others. The other comment that was made to support the idea was that if you were going to compete under circumstances of depletion. When does that happen?
There might also be an issue with glucose flow to the brain as a role in fatigue.
Another issue is the time of the long run. For Snell running 20 or 22 miles in 2 hours or so, there is less of an issue. For most of us not running at that pace, I suggest sticking with taking in CHO during training.
I will await any evidence that supports this idea of intentionally depleting glycogen. A woman has had one paper with 9 healthy (not trained) subjects suggesting that training on low muscle glycogen might up regulate mRNA transcription when it comes to protein syntheses. Not convincing and Professor Hawley did an effective job of shooting plenty of holes in the concept.