Michael Alello is about to run the Boston Marathon, but don’t call him an athlete.

He will permit the term runner — although even that gives him pause — but never athlete. The Boston race later this month will be the Catholic priest’s fifth marathon, not to mention the eight half-marathons he’s run. But for Alello, the image in his head of himself is the chubby band nerd, a trombone player in the Catholic High School marching band who had little interest in the sporting events where he performed.

“When you struggle with your weight, you always see an unhealthy person in the mirror,” said Alello, a 34-year-old Baton Rouge native. “That’s hard to change after many years.”

He was always overweight, at his heaviest ballooning to 275 pounds. At age 30, he set a goal to get healthier for no other reason, he says, than to stop being fat. Then parochial vicar at St. John the Evangelist, Alello joined Fusion Fitness in Prairieville intending to enroll in a spin class.

He never went.

Preaching one weekend about making commitments to prayer, spirituality and self-betterment, Alello turned to a trademark of his sermons. He personalized the lesson, telling the congregation of the spin class and the commitment he failed to honor.

Sitting before him in the congregation, as she did every Sunday, was Tiffany Lee, who happened to be an instructor of the Tuesday/Thursday spin class at Fusion Fitness.

“I come to Mass every Sunday and listen to you talk, and I’d like you to come to my class one time just to see if you like it,” she told Alello after Mass. “See you Tuesday at 9:15 a.m.”

“Basically, she called me out for what I had just preached,” Alello said with a laugh. “I couldn’t ignore that. I had to respond.”

Alello was there that Tuesday. The class, mostly female, became a community of faith, featuring spiritual and inspirational music as participants pedaled along. One day, Lee turned up the music, urging the class to go “faster.”

“What? You calling me fatso?” Alello called out, misinterpreting her words, earning the unflattering nickname Lee still uses today.

Since that first spin class, Alello has lost 60 pounds.

But eventually, he was transferred to St. Philomena in Labadieville, a small town in Assumption Parish, and his spin days were over. There are plenty of farms in Labadieville and plenty of wide open space but no spin classes. At the urging of his classmates, he took up running instead.

“Since then, it’s my sanity,” Alello said. “It’s what I do to clear my head, not only to stay healthy physically but mentally.”

He’s catered a parish mission “Holiness: A Lifestyle Change,” around his marathon training, asking participants to “rehydrate their lives” and “get into the race” for a holier life. Alello has presented the mission around the country, including two parishes just outside Boston that are eagerly awaiting his arrival for the running of the 119th marathon.

Boston has a qualifying time, one of the few marathons in the country to have such a requirement. Runners still wishing to compete but who don’t hit the qualifying time can apply to run for a charity and need to raise at least $5,000 for the team. Moved by the aftermath of the bombing at the finish line in 2013, Alello applied for a charity team in 2014 but didn’t make the cut.

This year, on April 20, he will run for the Leukemia & Lymphoma Society, having already raised $17,000 of a $20,000 personal goal. The cause is particularly poignant for the pastor.

He ran the 2012 Marine Corps Marathon with a cancerous nodule in his thyroid, diagnosed a month and half earlier in the midst of his training. Doctors told him of his condition, and Alello’s first question was whether the surgery could wait until he returned from the Washington, D.C., marathon.

“Here I was in the best shape of my life and had spent three years getting healthy,” Alello said. “I was very frustrated to all of a sudden be sitting in a doctor’s office to be told, ‘You’ve been doing all the right things, but you still have cancer.’ ”

The cancer’s gone. Now, his focus is Boston. He recalls sitting in his rectory fixated on the live stream of the aftermath of the 2013 bombing, befuddled why someone would attack “his sport.”

His phone was inundated with text messages and calls from parishioners, friends and workout partners who thought he may be there. Those images still etched in his mind, he prepares for Boston, hoping to break the personal best he set in Washington, D.C. More than that, though, he raves about the more than $900,000 he and his teammates have raised for those with cancer.

Openness is the cornerstone of Alello’s preaching. He’s shared his story with countless members of his congregations, recounting the difficult times when running and getting healthy tested his resolve.

“I relate that to my faith journey,” Alello said. “I can either be committed to my prayer or choose not to do it today. I can live for that day, or I can choose to let it pass me by.

“I inevitably always have a bad run while training. It could either be a 13-mile run or a 20-mile run. You just lace up your shoes and get back out there.”