To consult an almanac is one way to appreciate the magnitude of Ben Beach’s record streak of completing 51 consecutive Boston Marathons. Beach ran his first Boston Marathon at 18 in 1968, before the moon walk, Watergate and the fall of the Soviet Union.

A better way is simply to see him run. Beach, now 69, suffers from a rare neurological disorder that sends his left leg gyrating awkwardly, the lower leg extended sideways and nearly parallel to the ground with each stride.

“It’s shocking, really,” Beach admits. “But I’ve made my peace with that. This is what running is like for me now.”

Beach, from Bethesda, Md., will start Monday’s Boston Marathon and attempt to extend his record to 52. He has run with task-specific dystonia since 2002, when he was merely 34 years into his streak and tracking but always one year behind Pennsylvanian Neil Weygandt.

Weygandt retired from the race in 2011 at 64. The next spring, Beach equaled the mark and then passed Weygandt in 2013, the year of the bombings near the finish line. Along with thousands of other runners, Beach was still running when the course was closed. But marathon officials declared those beyond a certain checkpoint, including Beach, official finishers.

Running used to be less fraught for Beach, who ran his Boston personal best of 2 hours 27 minutes 26 seconds in 1981. In last year’s cold, wind and rain, Beach struggled to the finish in 5:46:59. Even while extending his record, the 26.2-mile distance these days is hardly the leader’s stroll up the 18th fairway at Augusta but more like an up and out of the trenches into no man’s land.

“Last year was tough, and more than twice as long as it took in the good old days,” Beach said. “That’s kind of pathetic and I’m disappointed my running is so labored. But all of us have to grind it out at least some of the time.”

With that self-effacing nod to his iron will, Beach is happier to acknowledge the element of fate in his remarkable run — last year, he found himself in the emergency room facing hernia surgery two weeks after Boston. This year, the week before this month’s Cherry Blossom Ten Mile run in Washington — Beach is the only runner to have completed every Cherry Blossom race since its inception in 1973 — left knee pain hobbled his preparation. Beach completed the race. “When it gets close to these two races, I tend to be a little more cautious and paranoid,” he said. “But over the years, there’s no doubt I’ve been extraordinarily lucky.”

Chris Bain, from Cabin John, Md., has a deep appreciation for Beach’s streak. Like Beach, Bain started his Boston streak while an undergraduate at Harvard. Bain now toes the line for his 22nd straight Boston Marathon. “I keep track of (Beach),” Bain said. “He’s exactly 20 years ahead of me. I’d love to think I can do what he’s done, but it’s hard to fathom, much less plan.”

Bain has been a prolific and talented local runner but now with three young kids and at age 41, his annual running focus is Boston. “I don’t know when (my streak) became a thing, but I just love everything about the race.” While Bain still runs well under Boston qualifying times — he finished in 2:42:56 two years ago — he is still three years away from Boston’s Quarter-Century Club and the relaxed qualifying time of six hours.

Beach has within his sights a new and greater goal: the record of 58 Boston finishes, held by marathon legend Johnny Kelley.

“I’m not another Johnny Kelley,” Beach said quickly. “He was an Olympian (1936 and 1948), won Boston (1935 and 1945) and finished second (seven) times. He’s an idol of mine for obvious reasons.”

But if Beach can maintain his streak through 2025 — six more years, when he would be 75 — he will have completed 59 Boston Marathons, the most ever. “If I can keep going, it’s within reach. But it’s harder to imagine now than it was 15 years ago.”

Some friends have suggested that Beach end the streak on his own terms and try something else. Because the dystonia limits his running to a few times a week, Beach swims, bikes and rows regularly.

Moreover, he has the unwavering support of his wife and three grown children, who have regularly accompanied him to Boston on his annual pilgrimage.