National Public Radio stalwart Linda Wertheimer ended 53 years at the storied outlet with a bittersweet note to her colleagues shared online the day after the 80-year-old co-host of “All Things Considered” announced her retirement on Tuesday. “I was incredibly lucky to arrive at NPR when I did, which was at the very beginning, and that’s what I want to talk about today,” her letter begins. “NPR was not yet on the air, ‘All Things Considered’ was barely an idea and nowhere near a program.” She goes on to discuss the many changes she experienced and responsibilities she held during her time at N…