The date was May 21, 1980. The Empire Strikes Back was #1 at the box office, Jimmy Carter was in the White House, Mount St. Helens was still erupting—and we were in Marlo’s parents house in Beverly Hills with 35 members of our families, taking each other’s hand in marriage.

This month, we’ll be celebrating our 40th anniversary of that memorable day. Thank you for your applause—we’ve earned it!

Like most couples, we vowed back then to love, protect, and cherish each other till death do us part. We also made a private promise that may seem odd: that we’d never work on a project together. Strange, right? We got along so well. But that’s the point—we wanted to keep it that way. We were both in high-pressure careers, and the last thing we needed was to bring that tension home. So we stuck with that game plan for 39 years, and it worked.

All of that changed last year when we were talking about our upcoming anniversary and decided to give ourselves a present—one that would both honor our big event and do something meaningful. That turned into an intriguing idea: a book that celebrated marriage, and not just ours but a lot of them. With so much negativity around us, we thought it would be great to sit down with long-married couples we admire, and through their stories, we could begin to uncover some of the mysteries of marriage.

So that’s what we did. Over the course of nine months, we crisscrossed the country, scheduling double dates with 40 celebrated couples (one for each year we’ve been married), including award-winning actors, athletes, comedians, musicians, newsmakers, and writers—and even a former U.S. president. In their homes and ours—over home-cooked meals, or bowls of nachos, or ample glasses of wine—these terrific husbands and wives shared with us memories of where they met, when they fell in love, and how they kept that first spark of attraction alive decade after decade.

But a surprising thing happened while we were diving deep into the rich stories of these couples’ lives together: we began opening up about our own marriage, sharing our stories as we never had before. And with each new couple we spoke to, our conversations made it clear that this would not be a how-to book—it would be a what book. What was that unique combination of work and play, and give and take, that created the magic that lay at the heart of their marriage?

Their stories were inspiring.

Chip and Joanna Gaines began their marriage with a strained checkbook and, amazingly, built a television and retail empire while also building a tight-knit family with five children.

Janice Crystal gamely took on the role of breadwinner in her marriage to Billy when he was a struggling comedian, all the while instilling in him the confidence that he would achieve the stardom he desperately sought.

Jesse and Jacqueline Jackson faced a heartbreaking infidelity deep into their marriage and found their way back to each other—all amid the turbulence of the nation’s civil rights battles.

Neil Patrick Harris and David Burtka and Elton John and David Furnish passionately embraced their chances to marry and raise children, exercising precious rights that they’d grown up believing would never be available to them.

Kevin Bacon and Kyra Sedgwick recovered from the horror of losing their life savings in the Bernie Madoff scandal, surfacing from the crisis even stronger than before (“The money was gone, but we still had each other,” Kevin told us).

John McEnroe and Patty Smyth, both fiery personalities, found a way to put their chaotic lives together and accept each other as “a work in progress.”

George Stephanopoulos and Ali Wentworth decided early on that the only way to ensure a healthy sex life during marriage was to keep doing it.

And Jimmy and Rosalynn Carter have relied on their faith to keep their bond strong. “We read the Bible together every night,” President Carter said, “and when I’m overseas, or when Rosalynn is traveling, we still read the same chapter, even though we might be 5,000 miles apart. We share the same text and it keeps us connected.”

The Carters were the first couple we interviewed for the book, and when we told them that we were writing this book together, Jimmy raised an eyebrow and smiled mischievously. “You know,” he said, “the closest Rosalynn and I ever came to divorcing was when we wrote a book together, and we fought about it every day. So y’all be careful, you hear?”

It took us 40 years to do it, but we’re still here—and looking forward to our 41st!

Author, actress, and activist Marlo Thomas and talk show pioneer Phil Donahue live in New York City. Their book, What Makes a Marriage Last: 40 Celebrated Couples Share with Us the Secrets to a Happy Life, will be released by HarperOne on May 5.