I've been doing treadmill running for over a year now. I've been slowly getting better at it, but my progress has been glacial. I see these sofa-to-marathon training plans and I just laugh. That may be for other people, but not me. My feet will start to drag or my pulse used to go way high and I would have to stop.
Once in a while I would be able to go further that I normally could do and was thinking that it was because of the time of day. Usually I run after work, but once in a while I would do it on a weekend mid-morning and I would do better then. Finally, finally I realized it was because I had eaten beforehand and not just a handful of skittles, but an actual light meal about an hour before like one of those mini burritos or similar.
I feel so silly that it took me this long to figure it out. Fill the tank first before you exercise, and give the food time to get into your system. Obviously, carbs help a lot, but I'm not entirely sure it has to be that specific though I haven't tried not eating carbs.
Anyway, if I eat beforehand I can run around 50% further which is a nice non-glacial result.