I would in part agree with some of the above regardind not feeling hunger or thirst.

My reasons may differ, but for me I would be to engrossed in the experience to pay attention. I have lately however been making a conscious effort to ask myself "am I hungry" "am I thirsty" which has not necessarily come naturally. Suffice to say I seem to be running better and with more enjoyment as a result.

Purely from personal experience.