I have read (and brewed) a number of beers using all extract or partial extract where they specifically call for most of the malt extract to be added at the end of the boil (either for last 15 minutes of boil, or even in cases of really light beers, at flameout, letting stand for 5-10 minutes with the lid on to pasturize). As mentioned above, you should have gotten better hop utilization, since the wort will be of a lower gravity, and you will wind up with a lighter colored beer not having carmelized the extract as much. Should be all good.