As somebody who only engages in "moderate" endurance activities (bicycle rides ~100 miles) I can relate the difference diet makes for me on long activities.
Endurance athletes speak of a thing called the "bonk"[0] - a point beyond which continued activity becomes much more difficult. This state is due to the exhaustion of glycogen stores in the body.
I've encountered this only a few times, but it's a really interesting phenomenon. In times I do not prepare well enough, I just run out of "energy." Everything gets harder, and I don't recover from it. It's not the case that I cannot continue, but continued exertion feels much more difficult. I feel mental fatigue as well, and I have felt "light headed" in this state.
This is why it is recommended to consume carbohydrates when engaging in endurance activities. It's not that you can't continue on ketosis, it's that you continue in a very suboptimal manner, and in something like cycling (where you are moving quite quickly) a sensation of light-headedness can be exceptionally dangerous.
Incidentally, the rate of depletion of glycogen stores is relative to the intensity of the activity. Operating further below the aerobic threshold will allow one to deplete the glycogen stores more slowly. If the OP is in good shape, this probably played a role in his success, as he might be running at a less intense pace than he would be capable of were he consuming carbohydrates.