Mike Link and Kate Crowley don't remember which of them suggested walking the 1,800 miles around Lake Superior, but somehow the idea stuck.
"It was pretty innocent to begin with," Crowley told MPR's Morning Edition, remembering the conversation she had with Link about what he would do when he retired.
Link, who was director of the Audobon Center of the North Woods for nearly 39 years, said it might be better now that neither of them remember who suggested walking around the entire lake.
"On that day when the black flies are really nailing us or there's a really bad storm, we can't point at the other and say, 'your fault,'" Link said.
On Thursday, the couple will start their trek from Duluth, and they'll be walking through the summer.
The black flies are one reason the two decided to go counter-clockwise on their trek -- the black fly hatch happens every 15 miles. Another factor to consider was the weather -- they'll be on the southern end of the lake when the weather is still cool and will reach the Canadian part during the warmest months.
Link and Crowley, who are both naturalists, will be doing more than just walking. The retirees figure they'll have to take a break every three miles or so, or about every hour. On all those breaks, they plan to take pictures in the four cardinal directions and take note of plants and animals they see.
"That becomes baseline data about what is existing right now in 2010 all around the lake," Crowley said. "That information can be archived for future generations and researchers."
Link said they are working with researchers from seven different universities to observe everything from invasive species to water levels to moose signs.
They also expect to see different levels of preservation along the shore: In Wisconsin, Michigan and Ontario, parts of the shoreline are considered National Park land. That's not the case in Minnesota, Link said.
"We have reached a point where we really have to think now about preserving a part of what is an extraordinary piece of heritage of Minnesota," he said.
Link and Crowley could be the oldest couple to complete the walk, and they believe they would be the first to do the walk following the shoreline rather than the roads.
Following the shoreline could take a toll on their bodies, especially walking the 600 to 700 miles of sand beach. But Link and Crowley said they're out to set an example of what can be done, even if it means reaching their limit.
Both said they're ready for the adventure.
"Every day we walk in a new spot. Every day we see something that we didn't see the day before," Link said. "How many times in your life can you have that?"
(MPR's Cathy Wurzer contributed to this report.)