Running is and always has been, at the most basic level, an exercise in endurance.
There is no special equipment necessary, no formal training either. Just pick up one foot, then place it down as you pick up the other. Then repeat. Quickly.
Do this for as long as you can stand the burning in your legs and the aching in your lungs, and you are a runner.
Is it any surprise, then, that in the wake of the bombings at the 117th Boston Marathon, several local running events have experienced increased interest? It's as if the running community is collectively saying, "This, too, we will endure."
From half marathons to 5Ks to charity walks, people have responded to the attacks by pledging to keep running. In some cases, doing so in record numbers.
"I was surprised at the overwhelming outpouring so soon after the marathon," said Steve Balfour, director of Boston's Run to Remember. "Says a lot about the determination and the compassion of the Greater Boston and running community."
The Run to Remember, which honors law enforcement officers killed in the line of duty, is a half marathon starting in Boston's Seaport District and winding its way through the city. In the days immediately after the bombings on Boylston Street, Balfour said more than 3,000 people registered for the race scheduled for Memorial Day.
"Last year we had 9,400 registrations, so we anticipated selling out at 10,000 this year," Balfour said. "However we anticipated selling out much closer to the race."
In previous years, the Run to Remember had never had more than 200 registrants in a single day. After the bombings, they had more than 700 in a day more than once before selling out at 10,400 entrants.
With people from far and wide looking for ways to honor first responders, victims of the attacks and people like fallen MIT Police Officer Sean Collier, the response for the Run to Remember makes sense. But it's not an isolated case.
Though he didn't have specific numbers for the event, JJ Larner said there's also been a noted increase in interest in the Quincy Half Marathon on May 5.
Traditionally a March event, used as a tuneup by many Boston Marathon runners, this year the Quincy race was postponed because of the weather. That meant a number of runners decided to withdraw, figuring they would be in no condition to run a half marathon so soon after Boston.
"Some of those opted back in after the atrocities, thinking they wanted to be out running even if they were below par," Larner said. "We feel it's important for everyone to continue living their American life, which in our case as runners is to go out and run a race somewhere. We are proud to welcome them to Quincy."
While participation as a whole is expected to be up only slightly in the Boston Marine Corps Honor Run 5K in South Boston on May 11, Lauren Proshan said there's been a noticeable increase in one category.
"Where we saw a shift rather was the number of people signing up in the 'active duty' category," she said. "We will have a significant increase in participation of those in the Armed Forces, local police and fire departments and a great showing from the 1-25th Marines Unit."
In 2012, the active duty category had 22 entrants. This year, there are already three times that many, with 66. And there are still two weeks until registration closes.
It's not just road races that have experienced an uptick in interest. The director of the Dana-Farber Cancer Institute's Boston Marathon Jimmy Fund Walk announced that the number of people who have chosen to walk the 26.2-mile Boston Marathon route doubled in the week after the attacks.
"Some who have registered have told us they are doing the Walk so that terrorism will not win," director Ann Beach wrote in a letter to Jimmy Fund supporters. "Some walk to honor those who were unable to complete the full 26.2 miles on April 15. Some walk for a cause close to their hearts, another insidious enemy: cancer."
In June, the BAA is scheduled to hold its 10K in downtown Boston. Starting and ending on Charles Street between the Public Garden and the Boston Common, the 10K route runs around the Garden and down Commonwealth Avenue and back.
Though he wouldn't speculate on interest level, BAA spokesman Marc Davis said the field right now will be at the previously set 9,500. Registration has been delayed one week, to Wednesday, May 8, as the organization continues to deal with the aftermath of the attacks.
Across the country, and indeed around the world, those tasked with securing mass sporting events have been forced to reassess in the wake of the bombings in Boston. By nature, road races represent a unique challenge -- long routes, many through urban areas, thronged with spectators can present many targets.
Details are still being finalized for the Run to Remember, including the exact route the course will take and what security measures will be in place, but Balfour said that's not unusual. Last year they didn't receive final approval until the week before the race.
"We're obviously putting a lot more effort into security," Balfour said. "There's no question it will be greatly heightened this year."
Sherry Betts is a school counselor in Leander, Texas, just north of Austin. She's trying to run races in all 50 U.S. states, and before the attacks in Boston had been contemplating a November race in either Kansas, Las Vegas or Massachusetts.
After the bombings, she made up her mind to enter the Myles Standish Marathon in Plymouth, Mass.
"I just decided it was the right thing to do," Betts said. "It is me doing my single part to send the message that the running spirit is not deterred and we're not afraid.
"We're not afraid to come to Boston and run these races, because running is who we are."
Jack McCluskey is an editor for ESPN.com and a frequent contributor to ESPNBoston.com. Follow him on Twitter @jack_mccluskey.