In the world of Hollywood, it’s rare to find a couple that is able to have a marriage that goes the distance and stands the test of time. That’s what makes the 40 year marriage between actress Marlo Thomas and former talk show host Phil Donahue such an inspiring rarity!

On Wednesday, the couple sat down with Ellen DeGeneres to reveal the secret behind their successful marriage as they promoted their new book “What Makes A Marriage Last.”

“It boggles the mind, nobody thought it would last, people thought we were so different that it would not last,” said Thomas, who is best known for starring on “That Girl.”

“Phil is very laid back and I am kind of like let’s jump on the phone and find a solution,” added the 82 year-old actress.

Thomas went on to say that the secret to a long-lasting marriage is all about the “desire” to stay together, and both people not having a plan B.

“In addition to lust, people who stay together are people who want to stay together,” she said.

Donahue chimed in to say that they should go on tours to talk about marriage before his wife offered more advice.

“If you get married thinking the other person will make you happy it won’t happen; the happiness comes when you are together,” Thomas advised. “Don’t marry potential, when you marry that is what you got.”

“The bottom of all of this is the love and the trust and the respect you have for the other person,” she continued.

Their new book tells the story of 40 celebrity couples, including Joanna and Chip Gaines as well as Michael J. Fox and Tracy Pollan. Thomas and Donahue told AARP The Magazine that they were inspired to write the book after they were shocked by the news of a close friend’s divorce, which led to them wanting to embark on a journey to find the “secret sauce” to a successful marriage.

“When we marry, we promise love and devotion, but no wedding vow can prepare us for the unexpected,” Thomas said. “We started to wonder if there really is a secret sauce to a successful marriage. And that’s when we decided to break an ironclad rule of our marriage – for the first time, ever – and decided to work on a project together.”