NEW YORK -- NBC News veteran Tom Brokaw said Friday that he is retiring from the network after 55 years.Brokaw, author of “The Greatest Generation,” was NBC's lead anchor at “Nightly News” and for big events for more than 20 years before giving way to Brian Williams in 2004.The 80-year-old newsman did documentaries and made other appearances for the networks after that, but he has fought cancer and his television appearances have been more sporadic.He said he will continue to be active in print