Dave McGillivray thought he understood struggle when he walked into a Jimmy Fund Clinic in Boston with a plan to run across the United States.

McGillivray was always short — picked last among his friends for basketball or baseball games. Cut from most of his school teams but determined to compete, he turned to running to combat the rejection and struggle that marked his adolescent athletic endeavors.

“Didn’t have to be picked, and no one could cut you,” he thought. “You just ran.”

He devised the plan to run across the United States in 1978 but needed more than just his personal fulfillment to endure the 80-day run spanning 3,542 miles. He needed a cause, something to make the run bigger than himself.

That led McGillivray to the Boston-based Jimmy Fund Clinic. He interacted with the cancer patients, mostly children.

Walking in the hallways, he noticed a sign.

“God made only so many perfect kids,” it read. “The rest of them have hair.”

“It almost changed my life,” McGillivray said. “They can truly feel hope and trust that they’re going to get better. That inner strength was infectious, and I needed to teach myself that, too.”

Now McGillivray spreads that message to others. He’ll be the keynote speaker at the Louisiana Marathon’s annual pastalaya dinner at 5:15 p.m. Saturday; tickets are still available.

Standing in Florida on Friday, he marveled at the runners passing him with Walt Disney World Marathon Weekend medals hanging from their necks. He spoke at a few seminars throughout the weekend, chronicling his life as a runner, Boston Marathon race director and philanthropist.

“What do you do if you have a bad race?” a woman asked during a seminar.

“There’s no such thing as a bad race,” he replied. “The fact that you woke up this morning is a gift. The fact you’re healthy enough to toe the line and hear the starter’s gun is a blessing.”

McGillivray admits he didn’t envision the path his life has taken. To date, he has finished 137 marathons. The 138th will come at some point Sunday, when he finishes the “Dopey Challenge” during Disney World Marathon Weekend — completing the 5K, 10K, half-marathon and full marathon in consecutive days.

His 1978 run across the United States began in Medford, Oregon, and ended in Medford, Massachusetts — his hometown. He split his days into 10-mile intervals, stopping to change shoes after each split. (He brought eight pairs in all.) Two years later, he ran 1,520 miles to raise more awareness and money for the Jimmy Fund, stopping at the White House along the way to meet Jimmy Carter.

The Boston Marathon’s race director since the 1980s, McGillivray notoriously runs the course at night, after all competitors have finished and his race director duties are complete. He ran the race blindfolded in 1982 to raise money for The Carroll Center for the Blind.

His birthdays provide no respite — not since his grandfather took him out for ice cream to celebrate turning 12.

McGillivray had a pond just outside his Medford home; it was six miles from his front door, around the pond and back home. Motivated, he ran the pond earlier in the day before meeting his grandfather.

“I need to burn this off,” he facetiously told his grandfather while finishing his ice cream.

So he ran the pond again — 12 miles for his 12th birthday.

It has been the same routine in his 48 birthdays since, increasing a mile each year to commemorate his age.

“Exponentially it gets more difficult because it gets longer and you get older,” McGillivray said. “I suppose at some time it will come to an end. I fool myself a lot and say, ‘All I have to do this year is one more, so what’s the big deal?’ It’s become one of those things. It’s a tradition.”

McGillivray has seen the shift in the running world first-hand. When he first started marathons, competition ruled the scene. It wasn’t about whether a runner finished; the question was in what place or how fast he or she got to the finish line.

“The walls of intimidation have crumbled,” he said. “People are believing in themselves now.”

It’s with that attitude that McGillivray continues to run. He’s not oblivious to his slowing pace and aging body, instead channeling the motivation and courage he had to return from his grade-school misadventures.

“(It) gives me so much resolve, confidence and esteem that I can do this,” McGillivray said. “I’m going to stop it when I can’t do it anymore. I don’t know why that is what it is.”

McGillivray fielded an array of questions at Disney World. The most obvious and most common: “Did you always want to do this?”

Now approaching 61, he playfully tells those who ask that he’s still not sure what he wants to do when he grows up.

That was until a few months ago. Driving down a highway, he read a one-word billboard.

“Accomplisher,” it said.

“Ah, there it is,” he said. “That’s what I wanted to be.”