Mark Gilbertson’s “big swim” across Flathead Lake may have been the most physically demanding feat he has ever pursued.

Considering that he’s almost 70 years old and he had triple bypass heart surgery just five years ago, he’s probably right.

Gilbertson set off Sunday at around 11 a.m. from a rocky shoreline at Angel Point on the lake’s west shore. After 6 hours and 11 minutes, he had swum an estimated six miles to emerge from the lake at Yenne Point on Woods Bay.

“It was more arduous than I thought it would be and that’s partly because of the waves, initially,” he said. “I was still pretty strong toward the end and I didn’t feel like I was totally exhausted until I got out and then I could hardly stand up. I did feel exhausted but exhilarated because I accomplished it ... I love that lake and I feel like I conquered it.”

The Seguin, Texas, resident has been visiting Flathead Lake regularly since he was 12 years old in the mid-1950s when his father and uncles owned a shoreline cabin. His brother, Phil Gilbertson, still has a place on the lake.

Gilbertson said he has been thinking about the big swim for several years and he has taken up regular lap swimming at Texas Lutheran University, where he was a philosophy professor before his retirement three years ago.

“I know the length of the lake has been swam by younger and hardier people,” he said. “I figured that (swimming the width) would be a special accomplishment, especially since I’m three weeks ahead of my 70th birthday.”

Wearing a wetsuit vest, Gilbertson initially encountered small whitecap waves, and found the open water to be more difficult than pool swimming or swimming close to shore. But he had the support of his wife, Pat, and his brother, Phil, and his wife, Carol, who were close by in a boat for the whole distance.

Phil said the group intended to go on a “beeline” to Woods Bay, a distance of about five miles, but a light surface wind caused the boat to drift south.

“We figure he swam about six miles because it was on sort of a sagging loop rather than a straight line,” Phil Gilbertson said.

Gilbertson balanced between a breaststroke and a crawl for much of the way, but finished up with the slower breaststroke. At the advice of his son, a long-distance trail runner, Gilbertson kept going with the help of high-energy gels.

“I was determined not to touch the boat the whole time,” he said. “If I would have had any assistance at all I would have been disappointed.”

Gilbertson said training and preparation were a key part of his big swim.

“I think this is the most perfectly arduous thing I’ve done, maybe in my whole life, but certainly since I was 50,” he said. “I would encourage any other senior who wants to do something serious like this to make the preparations and then go for it.”