ALLENDALE, Mich. - Tanner Berg could not have scripted his final moments as a Northern State track and field athlete any better. The Watertown native and fifth-year senior, who has written and rewritten the Northern record books more times than he can count, is a national champion. Berg claimed that title Thursday at the NCAA Division II outdoor championships in Allendale, Mich. It was a fitting end to a journey that began almost prophetically five years ago. “I came in as a little 18-year-old and didn’t know what I was gonna do with my life,” Berg quipped. “Mariah (Mougey) and Jon (Evenson) put a hammer in my hand and said, ‘You’re gonna be good at this.’ ” I said, ‘Well, we’ll give it a try, I guess.’ ” Mougey, who has coached athletes to the national stage 16 times, was pretty confident in her initial assessment. “I’ve been coaching at Northern for 18 years,” Mougey said. “When you know, you know. He didn’t believe me, of course, but