Even if they had to stop today, it would have been quite an accomplishment.
But Brad Ryan and his 89-year-old old grandmother Joy Ryan, who have visited 29 national parks so far, have no intention of stopping. Not until they've visited all 61 national parks.
The first one they saw, Brad told "Good Morning America," was the Great Smoky Mountains National Park in 2015. Since then, he and Joy Ryan have been to Badlands, Yellowstone, Glacier, Yosemite, Joshua Tree, Grand Canyon, Zion and Mammoth Cave, among others.
"I remembered a prior conversation with my Grandma Joy when we discussing my 2009 Appalachian Trail hike from Georgia to Maine and she expressed regret that she had never seen the mountains and explored the great outdoors throughout her life, so I invited her to join me on my weekend escape to the Smokies," Brad told "GMA."
(More: How and why you should go on a digital detox vacation)At the time, Brad was a a fourth year veterinary student at The Ohio State University College of Veterinary Medicine, but needed a break.
"I needed an escape from campus during my clinical year after news of a second year vet student's suicide was made public," he said.
There's been a lot of laughs and a few hiccups along the for the duo.
A highlight for Brad was "watching my grandmother tap into her inner child as she rolled down a sand dune at Great Sand Dunes National Park and Preserve at age 87." So was being trapped in an hours-long traffic jam of a bison herd in Yellowstone's Lamar Valley, and watching the sunrise over the Grand Canyon.
Less fun? "Being charged by a moose along the shores of Fishercap Lake in Glacier National Park," he said.
There was also the time Joy had vertigo at Biscayne National Park in Florida. "She still managed to take a speed boat to Boca Chita Key and climbed the spiral staircase of an ornamental 65-foot lighthouse on the island," Brad said.
(More: This over-water hammock meets all your Instagram needs this summer)The duo hopes to visit the remainder of the parks sometime in 2020 to celebrate Joy Ryan's 90th birthday.
"We have received a lot of support from around the world so far, and I am hopeful that opportunities will arise to make it feasible to travel with Grandma Joy to the most remote US National Parks," Brad said.