President Biden acknowledged in a new interview that he might not have had the vitality to serve another four years in office, even as he insisted that he could have won re-election had he stayed in the race. Mr. Biden, 82, who abandoned his bid for a second term in July under enormous pressure from Democrats panicked over his faltering debate performance against former President Donald J. Trump, maintained that he was still in good enough shape to run the country as he finishes his term.