What makes the Boston Marathon worthy of all this effort? Why are so many people like me determined to run it, particularly when there are countless other races to be conquered, the paths to them much easier and shorter, in some cases as simple as a click of the mouse? There are marathons large and small, quaint and legendary, flat and hilly – enough experiences to challenge, inspire and fulfill a runner for a lifetime, without ever travelling to Hopkinton, passing through Ashland and Framingham, climbing Heartbreak Hill, chasing down the famous Citgo sign and crossing the fabled finish line on Boylston Street.

You could try the fabulous New York City Marathon, for example. No other race in the world gives you the experience of crossing the Verrazano Narrows Bridge, travelling the five boroughs with two million spectators cheering you on, hitting the wall of sound on First Avenue and crossing the finish line in Central Park. I’ve done it three times and I’d go back in a New York minute.

There are plenty of other particularly well-organized events. I’ve finished the Walt Disney World Marathon, which takes you through Epcot, the Magic Kingdom and other Florida theme parks, and the Marine Corps Marathon, a tour of all of Washington’s historic landmarks. Which organization – the U.S. Marines or Disney – has more control freaks who excel at logistics? It’s too close to call.

And I have dozens of other races on my wish list. I’d love to run scenic Big Sur, the regal London Marathon, the flat course in Berlin where world records are routinely set. There are events known for their energy and emotion, like the Rock ‘n’ Roll Marathon in San Diego, and picturesque settings like Honolulu. There are big races in Chicago and Philadelphia. There are well-regarded smaller marathons in Duluth and Pittsburgh.

If an urban race isn’t interesting enough for you, you can run a marathon on the Great Wall of China or the frozen rock of Antarctica. There are even longer events, like the 56-mile Comrades Marathon in South Africa, the oldest ultramarathon in the world, and multi-day races like the Yukon Arctic Ultra.

And yet for me and for millions of other runners, no other event measures up to Boston. Ask someone to name the most prestigious marathon in the world, look at any comparison of the top destination races, and it’s almost unanimous: Boston is at the top of the list.

What’s so special about Boston? For one thing, it’s the oldest marathon on the planet, launched in Victorian times, when McKinley was president, veterans of the U.S. Civil War still numbered in the tens of thousands and Thomas Edison had only recently founded his first electricity-generating station in New York City.

The Boston Marathon traces its roots back to the very earliest days of competitive long-distance running, almost thirty years before any other current marathon was created. It’s unlikely that anyone who ran Boston in 1897 was still alive when the first New York City Marathon was staged seventy-three years later.

In more than a century of Boston Marathons, naturally, there have been dozens of historic moments. Johnny Kelley’s heartbreak in 1936. Kathrine Switzer’s daring entry in 1967. The legendary “duel in the sun” in 1982. Rick and Dick Hoyt’s first of more than thirty races together in 1977. And, of course, the tragedy of the bombings on Boylston Street in 2013.

But there is more to Boston than just its vintage and its heritage. It is also exclusive. It is the race with a long lineup, not just a starting line but a guarded entrance. The gatekeeper is both fair and ruthless; only those who have earned their place are allowed to enter.

It is not about luck, apart from the benefit of good genes. A well-timed click in the first minute after registration opens will not get you to Hopkinton, nor will a providential entry in a lottery. You can enter through a very limited number of charity spots or travel packages, but the vast majority of participants receive a much-coveted invitation through the front door, making Boston the most elite, undemocratic and prized of all the marathons. By insisting on qualifying times, it is both cruel and inspiring, judgmental and alluring. Unlike any other race, a Boston entry is a validation of more than just a choice, but some combination of genetics and industry.

For a fortunate cohort of runners, Boston’s qualifying times are well within reach. Some people qualify in their very first marathon. I am both deeply envious of and profoundly sympathetic to them. To win the lottery the day after signing your first mortgage is a stroke of good fortune that makes life instantly easier. No one would turn down the freedom. But those lucky souls are denied the value and satisfaction of a lifetime of working hard toward an ambitious goal.

For only the fastest runners is qualifying a routine accomplishment. To the rest, Boston remains a dream, resting tantalizingly on the spectrum somewhere between possible and impossible. Like many things in life that are just out of reach, it becomes an obsession. It’s the oasis on the desert horizon, the chocolate éclair in the shop window.

Few runners think of qualifying in their first marathon. Typically, the only goal in a maiden voyage is to get to the finish line and find out if you have what it takes to cover the distance, regardless of how long it lasts. But after that, at some point early or late in a marathoner’s career, a measurement is taken: how far am I from getting into Boston? How much older and how much faster must I become? What will it take to get there? You might try to put it out of your mind until you are of the right combination of age and speed to have a reasonable shot. But it doesn’t help that you talk to dozens of people who have run Boston and rave about it. Sometimes they are wearing a brightly colored Boston Marathon jacket when they do so.

And the more you hang around other runners, the more you get asked, “Have you run Boston?” and you have to explain that no, you haven’t qualified yet, not even after six marathons, or a dozen, or more.

For many runners, then, it becomes an itch that must someday be scratched. You must attempt it, as the explorer George Mallory once said of climbing Mount Everest, because it is there. And so it becomes a question of when and how, not if. You cannot be satisfied until you’ve earned your place in Hopkinton and run to that famous finish line in Copley Square.