“I’m not sure it’s possible to walk or ride a mule through the Sierras. No one has been on the old El Camino Real since before God was a baby.”

It was supposed to be a warning from someone experienced in the ways of Baja California’s desolate, rugged terrain, in a region that Jesuit and Franciscan missionaries had struggled to establish Catholic outposts more than three centuries ago.

But to Edie Littlefield Sundby, them’s was fightin’ words.

“The only time I feel truly alive,” she told a friend, “is when I’m outside walking with God along the old mission trail.”

Even if it was 800 miles, from Loreto to San Diego. Even if large portions of it were not just desolate, but dangerous, barely fit for walking by any two- or four-legged creature. And even if she didn’t have stage 4 lung cancer, a disease that, a doctor had told her upon her original diagnosis 10 years ago, would likely cause her death in three months.

Long story short: In 2013, this slightly built, one-lung-ed Oklahoma native had walked the 800-mile U.S. portion of El Camino Real, from San Diego to Sonoma, in 55 days. Two years later, after learning “the old mission trail” had actually originated in Mexico, Sundby completed the second half of her journey, enduring extreme weather and, on occasion, extremely unpredictable assistance from a series of vaqueros hired to aid and accompany her.

The complete story is told in “The Mission Walker,” Sundby’s new book released in late July (Thomas Nelson Publishers). It is a tale of adventure, sometimes hilarious and often harrowing, but it is above all a testament to the power of faith.

It was, she says, “a walk on holy ground.”

At home in La Jolla, where she attends St. James-by-the-Sea Episcopal Church, Sundby still marvels at what God has helped her accomplish — not just walking 1,600 miles, not just staying alive years longer than had been predicted, but opening her eyes, mind and heart to possibilities attainable through faith and determination.

“God is mystery — in fact, life is mystery,” she noted in a recent phone interview. “And even with everything we’ve learned and know about cancer, it’s still a mystery. And so what do we really have to worry about?

“In AA, they say, ‘Let go and let God,’ and the most amazing things can happen, even if they aren’t necessarily the things what we want or what we think should happen. And you know what? That’s OK. In Philippians, St. Paul tells us, ‘Do not be anxious about anything.’ In other words, embrace the uncertainty; it strengthens our faith. So when I decided to do this walk, there wasn’t a nanosecond of hesitation on my part.”

In truth, there weren’t many nanoseconds to waste, once Sundby decided to make the Mexican portion of the walk that St. Junípero Serra took three months to make in 1769, right before he founded Mission San Diego de Alcala.

Because earlier in 2015, two years after she’d walked from San Diego de Alcala to San Francisco Solano, a CT scan showed the return of cancer, in her (remaining) left lung.

“When cancer comes back, the timing is never convenient, and you don’t have time to do things in an orderly fashion,” she said. “But that’s how I’ve lived much of the last 10 years: you’re never totally prepared for what might come, only to expect the unexpected. And that’s a grace. It propels you into action.”

She met with historian Harry Crosby, who years earlier had made numerous mule-pack trips in the region and possessed the best (in some cases only) detailed maps showing what trails existed. She studied old missionary journals and gold prospectors’ diaries, consulted with experts on the area (several of whom questioned her ability, if not her sanity), and equipped herself with necessary survival supplies (including her medications and a satellite phone).

And on Oct. 25, 2015, the 318th anniversary of Mission Loreto Concho’s founding by Italian-born Jesuit Father Juan Maria Salvatierra — “a man of optimism and action” — Sundby departed from Loreto, accompanied by the first of many vaqueros who lived in and knew the Baja’s backlands, some admittedly better than others.

“I didn’t know much Spanish, or a mule from a donkey,” Sundby chuckled. “But the experience was so brutal, so wild, that it got my mind off the cancer. In that way, the walk was both healing and empowering. There’s nothing so exhilarating as walking next to or being on a wild mule in the desert, who isn’t sure he wants to take this rugged trail any more than you do.”

Vaqueros, mules, horses and supplies came and went. So did wind, rain and blistering desert heat. There were many times when Sundby felt physically and emotionally battered and spent, and yet each time she rebounded by embracing the challenges as graces from God.

“In the wilderness,” she observed, “you are so attuned to the moment that you become totally in awe; you experience a oneness with God, and you really rely on God to get you through the challenges. Think about how Jesus and John the Baptist spent time in the wilderness and the desert. They must have had horrendous challenges just meeting basic needs, but they didn’t worry about where their next meal was coming from, or where they would sleep. So given that, I could deal with the challenges.”

What did her family — husband Dale and adult twin daughters — think of her enterprise?

“I have a unique family,” she explained, “because we’ve been dealing with my cancer, my treatments, the what-ifs, the grief, for 10 years now. They’ve been through it all, they know what taking these walks means to me, and their faith has been strengthened as well. And that makes life a greater gift for all of us. So they understood why I needed to make this journey.”

The month before starting out from Loreto, Sundby returned to El Camino Real to walk the 50 miles from Mission San Juan Bautista to Mission Carmel, arriving on the day Junípero Serra was canonized in Washington by Pope Francis.

“I felt very blessed to be following in the footsteps of a saint in this journey,” she said. “The missions are our heritage; they are tremendously vital to the well-being of our state. And I believe Serra belongs to more than Catholics, to more than California; he is a saint for all of us.”

Father Serra, too, had health challenges during his journey, a reality that helped Sundby focus on walking each step, one at a time, “always forward,” in the spirit of the saint.

“With an incurable disease, the end of your life speeds up, so to speak, but walking slows life down like nothing else,” she said. “And the nice thing about cancer, if you can call it that, is that it gives us an opportunity to reflect, to heal relationships, to renew our faith in God, and to see God in absolutely everything.

“I’ve never seen myself as a cancer survivor; I see myself as a hostage under siege, and that brings out the fighting spirit in me, it helps me fight this battle more effectively. Because you never know when it’s coming back.”

She smiled. “So every day I am here is a bonus day, thanks be to God.”