He’s taken on murderers, robbers, drug cases and sex crimes, but Detective John Roe is nervous about his latest challenge — “fighting the boredom” as he begins his retirement after 44 years in the NYPD.

The city’s longest-serving police officer, Roe, 62, was wildly applauded by about 100 active and retired cops as he made a ceremonial “walkout” at Harlem’s 26th Precinct station house yesterday amid bagpipe music.

“I wanted to be a cop in the first place because my whole family was — my dad, brother, uncle, then nephew,” said Roe, who’s also the oldest of the NYPD’s 34,000 officers.