The wall is not a good place.
And if you run marathons, the wall is the worst place.
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It's why you carbo-stuff in the days before the race, to store more glycogen, which provides energy. When that is gone, the body goes to work eating fat, and eventually it feels like you're devouring your own muscles.
That's the wall, most frequently hit about 18 to 20 miles into a marathon. The rest of the journey can be extraordinarily painful.
So it's not uncommon for runners to say you feel like you're dying, an unfortunate choice of words probably used by some participants of the Boston Marathon only minutes before three people were killed and nearly 180 injured in a terrorist attack Monday.
As brutal as a marathon can be, it is incredibly satisfying to finish, the Mount Everest of running for amateurs.
In Boston, it is a holiday. It is Patriots' Day. It's a family day, when families gather to cheer on their children, siblings and parents in one of Boston's most cherished events.
And for hundreds of families affected by Monday's terror, life will never be the same, as lives and limbs have been lost, emotional and physical pain destined to last forever.
One family has been literally torn to shreds. If reports are accurate, 8-year-old Martin Richard is dead, his mother, Denise, is in critical condition with a brain injury, his sister lost a leg, and they were all there with another child, a boy, to watch their father, Bill, complete the race.
A conflicting report Tuesday out of Boston said Bill Richard did not run and that the family was simply together enjoying the day. Either way, that will never happen again.
A family in ruins only because they wanted to spend a holiday as a family, living and laughing and holding hands.
"He was a great little kid, full of life,'' a neighbor told The Boston Globe, speaking of young Martin. "Always smiling.''
"We always played on Fridays,'' 8-year-old Kaitlyn Lynch told the Globe. "We draw together. We draw sports pictures."
Flowers and stuffed animals crowded the steps to the family's home in Dorchester, Mass. Friends lit candles. Strangers stood and wept.
Neighbor Dan Aguilar told the Globe that the Richard family was close, and that Martin and his brother were almost always in the family backyard -- regardless of weather -- playing soccer, hockey, or baseball.
Neighbors talked about seeing the family on Easter Sunday when they played outside, drawing butterflies and flowers with chalk on their driveway, where the pictures remain today.
"They are just your average little boys,'' Aguilar told reporters near the Richard home. "They are a good family. They are always together.''
Among the many thousands who will never know why they escaped death or serious injury by only seconds is 78-year-old Bill Iffrig, who was running in his third Boston Marathon.
You have undoubtedly seen the video of an older runner blasted off his feet, only to be helped up after scraping his knee. Bill Iffrig walked the final 10 yards for a time of 4:03:47. The image of him on the ground graces the cover of this week's Sports Illustrated.
"I ended up second in my division,'' he told The Herald of Everett, Wash. "After you've run 26 miles, you're not going to stop there.''
That's what runners do. They run and they finish. It is an odd sport that is nine parts pain and one part satisfaction, the happiness a result of a job completed, a personal best, a mountain climbed.
Bill Iffrig got up. That is one of life's few guarantees. We are given the opportunity to get back up. We get knocked down, but we -- as people, as children, as parents, as Bostonians, as Americans, as runners -- can get up again.
The older you get, the more life feels like a sprint rather than a marathon, but we don't lock ourselves in our homes, and we don't give in to the calendar -- or fear.
So if you always run Boston, show up next year and run hard again in memory of the Richard family.
If you run Chicago, run this year and raise money for charity like you do every year.
Above all else, don't stop running, don't stop living, don't stop pushing through that wall.
Think about little Martin Richard and what remains of his family.
And never let a day go by without hugging the ones you're with.
•Listen to Barry Rozner from 9 a.m. to noon Sundays on the Score's "Hit and Run" show at WSCR 670-AM, and follow him @BarryRozner on Twitter.