The Martins' tone may be lighthearted, but their message is serious. "Our big mantra is 'postpone nothing,'" said Tim. Anytime is a good time to travel, "as long as you are healthy," which can be unpredictable as people age, he said, "so postponing is a mistake."

The couple also stresses the importance of making practical decisions.

"We didn't spend all of the money from the sale of a house traveling around the world. That's not what we're doing," Tim said. "We still live on the same stipend as we did before we sold the house. Nothing has changed. We are being fiscally conservative," he said, budgeting carefully and working closely with a financial adviser.

It hasn't all been easy. In Buenos Aires, for example, detailed in chapter four of the book, Lynne recounts problems that made them leave early. "It made us question our tolerance and flexibility, and if we were able to live this life," she said. There were mistakes along the way, too, but they were learning experiences. When they started out, there was no blueprint or road map, and they relied on trial and error. "Everything has been valuable," she said.

After the book promotion, the couple will head to Paris for a few months, beginning in June, followed by a visit to family in California in September. October will find them back in San Miguel de Allende, where their travels began nearly four years ago, and then off to Ecuador. In 2015, they plan a big trip to South Asia. They plan to travel to less challenging locations, like within the U.S., as they age.

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"It's isn't all pretty, but it's all been rewarding," Tim said, noting that their richest experiences have been personal encounters along the way. "It always comes down to people."

"This has been quite a journey," Lynne said. "It's been fun. It's been really fun."

By Tanya Mohn, NBC News

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'Nomadic retirees' travel the world