It took a radio disc jockey to remind Jay Leno where he’ll stand in “Tonight Show” history when he walks off the stage for the last time Friday.Leno was at the wheel of one of his famous vintage cars when he heard the DJ conduct a pop quiz: Who’s the second-longest-running host of “Tonight,” after Johnny Carson?  “The guy on the radio actually got it before I did,” Leno said Thursday, smiling. “It just sort of made me laugh. I went, ‘Oh, that’s pretty good.’”  Maybe even better than good? “I come from ‘pretty good,’” replied Leno, unfailingly modest in interviews. “If somebody wants to say even better, that’s great.”  He will have posted an impressive 17 years as “Tonight” host, but well short of Carson’s three decades that ended with his retirement in May 1992. Leno debuted as “Tonight” host a few days later.  He leaves the show atop the late-night ratings, his run abbreviated by NBC’s decision five years ago to create a succession plan that gives “Tonight” to Conan O’Brien.  On the eve of his final two shows, Leno strikes an unsentimental tone. But he knows what he’s leaving behind as he moves to a new, untested 10 p.m. EDT daily prime-time show for NBC this fall.  “Will I miss it? Yes, terribly. It’s the most wonderful job ever in show business,” he said of “Tonight,” which started in 1954 with Steve Allen as host.  Unlike the solitary road life of a typical standup comedian, telling jokes to an audience of maybe 100 or so, Leno said, he had the chance to make millions of viewers laugh — and then go home each night to his wife.  In a conference room at NBC’s studio, a bulletin board typically filled with lists of guests and comedy bits for upcoming shows is nearly bare, down to the final two shows. “Prince,” read one red card for Thursday, when the pop star was set to appear.  Prince’s baby-blue Bentley was parked in the studio lot, near the backstage entrance and next to one of Leno’s prized vehicles, an eye-catching red pickup truck.  In the lunch room, the staff raided boxes of snack cakes, topped by a sign indicating they were compliments of Lyle Lovett, a guest earlier this week.  On Thursday’s show, Billy Crystal, Leno’s first “Tonight” guest 17 years ago, returned to salute him with a musical medley akin to Crystal’s Oscar ceremony opening numbers.  “Mustangs and Mazdas and shiny Mercedes, Model T Fords that he steals from old ladies,” sang Crystal, to the tune of “My Favorite Things.” Starting Monday, at a newly built studio at nearby Universal City, O’Brien will be the man in charge of “Tonight.”  Leno declined to give advice to O’Brien, whom he called “a terrific guy” and a friend.  “He’ll bring his sensibility” to the show, Leno said.  Over the years, the two have called to commiserate privately after a “dreadful” guest visited “Tonight” or O’Brien’s “Late Night,” Leno said.  “Hopefully, we’ll continue to do that,” he said. After all, the two are enjoying an amicable transition.  “That’s what’s great about these American democracy things. We can peacefully hand over talk shows without looting and rioting,” Leno joked.  This summer, he’ll continue doing his standup appearances that filled his weekends during his “Tonight” reign. More importantly, he’ll get ready for the new show that, he acknowledges, will face stiff competition in prime-time.  “It’ll be really tricky. But we’ll just do the best we can,” Leno said.  NBC is owned by General Electric Co.