“I was a real smoker, man — die-hard,” Mr. Carl said. “I’d wake up at 5 in the morning to go to the bathroom and suck down three or four of them.”

Feeling flabby two years ago, which may or may not have something to do with a breakup with Lady Gaga, he went in search of a diet guide, but, he said, “all the books had a person with extremely chiseled everything on the cover.”

“I’m not the abs guy,” he said. So he decided to write his own fitness and diet manual.

His secret to losing 40 pounds (he has since lost another 15) was running, which he does, logging 50 to 75 miles a week. He completed four marathons last year and ran a half-marathon during a snowstorm in January in Central Park, finishing just behind a man whose left eyelid was frozen shut.

“It’s such a high, but it’s a drug you have to earn,” Mr. Carl said after that race, icicles dangling from his mutton chops.

Still, the fitness regimen hasn’t kept Mr. Carl from pursuing a good time. He and his friends appear like time-warp denizens of the Sunset Strip, circa 1986, where life’s major concerns were looking good, meeting girls and scoring Crüe tickets.