COLUMBUS, Ohio — Mitt Romney is a man of steady and immutable habits. He drinks Cherry Coke Zero. He eats peanut butter and honey sandwiches. And he works out on an elliptical machine.

Day, after day, after day.

But as the bromance between Romney and his Republican vice-presidential choice, Rep. Paul Ryan of Wisconsin, blossoms, even his most treasured rituals are open to tinkering at the hands of his younger — and fitter — running mate.

Romney said Wednesday that, after years of arm-and-leg swooshing elliptical monotony — he occasionally hops on a treadmill, too — he is willing to experiment with Ryan’s favored exercise routine, a sweat-inducing and muscle-confusing regimen known as P90X, which involves weights, rubber bands and scary-sounding exertions like “Ab Ripper X.”

“I might have him show me how to do it someday,” he told the hosts of CBS’ “This Morning” during an interview, confessing, “I have never tried that.”

In what seemed like a case of workout envy, Romney observed, “That workout of his, he’s in pretty good shape.”

When the interview wrapped up, Romney headed to — where else? — the gym of a hotel in downtown Columbus. True to tradition, he jumped on a Star Trac elliptical machine and swooshed back-and-forth for 45 minutes.