I based my last commenton a logical explanation from a friend:
-Undead have no CON. The MM states clearly that this is very different from having CON 0. It also states that a creture without CON *always* fails a CON check (MM p. 10).
So, that could be understood like this:
If one can run for a number of rounds equal to his/her constitution score, then a creature *without CON* can´t run at all (no CON means no running without a check, when check is made, it automatically fails).
I think that this makes sense because I can´t picture a creature trying to use more power from a system of organs *that don´t exist* to move faster.
But, even if you can picture this, then i think that running forever because they *don´t need* to make CON checks is little out there.
Also, if it were (was?) right to suppose that because they are CONless they don´t need to make CON checks at all (implying running forever ability and more) i believe the MM or DMG would state so, instead of saying that they *always fail CON checks*, which clearly implies that they *do make CON checks*, whenever necessary.
So my final opinion is that while I think that everything I said makes more sense, I *can* picture a *vampire* running (because of movies and stuff like that, but thats just a feeling, and because I am sleepy again) and I think that this is something that should be covered in the FAQ because it still seems a little gray to me. I looked for it and didn´t find anything.
PS: We also have to remember that undead use CHA instead of CON only in very specific situations, described in the MM or other books. Example: Concentration checks for spellcasting (in the MM).
They don´t have the benefits of *two* ability scores (CON & CHA) in just one (CHA). That´s a wrong logical conclusion.