Stretching wont affect endurance either way. If it had to go one way or the other it would increase endurance because the greater flexibility would mean less effort necessary for movement. But I doubt it makes much difference. I've never seen any studies on this either way. The only thing I've seen stretching affect is maximal or near-maximal efforts in muscular strength/power. But that's only been if the person engaged in a long period of stretching right before the event. And that's not set in stone either.
