April, 47, a tour guide at the statehouse in Trenton, came across the professor's work five years ago. Living in Fishtown, a bit at loose ends, the rest was obvious.
The goal: to further "responsible running and consumption in the interest of science." Translated: a three- to five-mile weekly run to a cool bar for craft beer, a venture buoyed by camaraderie and rigorous academic underpinnings.
Meanwhile, in Spain, it turns out, the professor had been having trouble getting people to notice his scholarship, which was grounded in research, though inspired by tennis.
Before he knew it, April became Exhibit Uno.
Invited by Castillo to act as a run ambassador at a conference in Madrid, April appeared this fall on a panel with former Spanish Olympic team point guard Juan Antonio Corbala, now a cardiologist. In October, Corbala and actor/heartthrob Santi Millan, both beer ambassadors, got a write-up in GQ Spain.
"Millan kind of attracted the media," April said. "GQ interviewed him. When I looked at the magazine, the first two paragraphs were all about me and the Fishtown Beer Runners."
"Beer Runners es una iniciativa promovida por cerveceros de España y la idea surge de Fishtown Beer Runners, un club de corredores de Filadelfia que . . ."
And so April - now a globally celebrated fundador of the Fishtown Beer Runners - returned home to Fishtown triumphantly to continue to prove the professor's theory weekly.
The thing caught on.
The professor was grateful. "He's so tickled by that," April said. "He said he must be the healthiest man alive because every Thursday 70 people toast to his health."
This past Thursday, the group joined up with the South Philly Striders for a "Christmas Lights" run that collected food for Philabundance.
In the crowd was running author/guru Christopher McDougall, who gave an early benediction to the club's mission. McDougall says regardless of science, the beer runners have it right: Running does not have to be solitary torture.
"People have such run anxiety; what's cool about this is there's none of that," he said. "As soon as I heard about Fishtown Beer Runners, I thought: The world's heading in the right direction."
As they gathered at Broad and Oregon, Jim Benson, 31, a Web designer, affirmed his creed. "I do believe in the professor," he said. Later, he would affirm it with a Twin Lakes Tweed Stout. "Every week, we run for his theory and prove it."
Others were proving the professor's theory for years before they even knew it. "I've been running and drinking for years," said Rich Lucas, a recruiting manager and screenwriter.
Joy Shank, 37, a nurse from Fishtown, stood by the science. "What I can tell you is, you also get drunker on less beer after running," she said.
Running through the streets of Philadelphia every week, she says, makes her want to call out to those they pass, "Put down your cigarette and come run with us."
But she also broke ranks by having both a glass of water and a Kenzinger in front of her. "David might get upset with my saying this, but they usually give us water at the bars."
She said April had accomplished a lot more than his unlikely foray into Spanish GQ. "He's brought together so many people," she said. "People who like to run and drink beer - what's better?"
John Longacre, owner of the American Sardine Bar, said the beer runners were naturally popular with bar owners, especially ones like him who own cool bars in out-of-the-way places.
"David is very good about taking people to a place that needs a new set of eyes on it," Longacre said. "Some guy from Marlton probably wouldn't be looking for craft beer in Point Breeze."
Perhaps most in need of the finish-line beer was Dave Maver, 26, an intensive-care nurse dressed as Rudolph, who led a group of runners dressed like reindeer. Jen Leung as Baby Jesus blocked traffic along the route, which meandered from Broad and Oregon to 18th and Federal.
The narrow sidewalks and one-way streets posed a bit of a challenge for the sleigh and its connected reindeer, but all of it made people smile.
"Looking good," said Brian DeSauter, a visitor from Connecticut outside the King of Jeans on Passyunk Avenue.
"Why are they doing this?" asked hairstylist Stephanie Dougherty as the group passed John Paul of Philadelphia Salon at 13th and Wolf.
"We'll see you at the bar," called out Jim Cimorellion on East Passyunk, who'd been directing confused runners (some people assumed they were on a pub run and had already imbibed) and knew the drill. "It's cute."
The reindeer group soon lagged behind, so much so that Baby Jesus broke away. "I'm afraid they will run out of beer," she said.
But like some Fishtown Beer Runner Christmas lights take on the Hanukkah oil story, running Baby Jesus had nothing to worry about. The beer at American Sardine Bar lasted for eight reindeer.
Contact Amy S. Rosenberg
at 215-854-2681 or firstname.lastname@example.org. Follow on Twitter @amysrosenberg.