Author, endurance athlete and former Navy SEAL David Goggins is famous for saying, “The pain that you are willing to endure is measured by how bad you want it.”

He’s finding out how badly he wants it this month, as Goggins is back in the U.S. military and undergoing the mentally and physically challenging introduction to pararescue training at Joint Base San Antonio-Lackland.

The Air Force confirmed that Goggins, 51, has re-enlisted and is taking part in training at the base. It did not say when he entered training.

“Air Force Master Sgt. David Goggins is currently assigned to the Special Warfare Training Wing,” said 1st Lt. Casen Salitore, a spokesman with the training wing. “In accordance with wing policy, we do not discuss the status, progression, or details of current trainees.”

Goggins has gained fame as an endurance athlete, speaker and social media presence. He has completed more than 70 ultra-distance races and is a former Guinness World Record holder for completing 4,030 pull-ups in 17 hours, according to his website. He has written two books, “Can’t Hurt Me” and “Never Finished,” and often speaks of how he overcame a childhood filled with poverty, prejudice and physical abuse. Outside magazine dubbed him ‘The Fittest (Real) Man in America.”

“Through self discipline, mental toughness and hard work, David  transformed himself from a depressed, overweight young man with no future into a U.S. armed forces icon and one of the world’s top endurance athletes,” his website says.

Goggins previously served in the Air Force from 1994 to 1999, and entered the pararescue training program. However, he was diagnosed with sickle cell trait and medically removed from the program, according to the military.  Goggins instead served in the Air Force Tactical Air Control Party.

After leaving the Air Force, Goggins enlisted in the Navy in 2001 and served until 2015. He graduated from Navy SEAL training, serving on Seal Team 5, according to his military bio. Goggins also graduated from Army Ranger training, according to the military.

Reports that Goggins had returned to the Air Force for another crack at the  pararescue program’s coveted maroon beret have circulated on social media in recent weeks.

A post on the site Come and Sit Down, an apparent acronym for the military’s Combat Search and Rescue, purports to show a photo of Goggins standing with a group of airmen, with the photo captioned, “David Goggins going through pararescue again.”

Goggins has not spoken publicly about why he decided to make a second try at completing the Air Force’s pararescue training, which can take as long as two years to complete.

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Military officials declined a San Antonio Express-News request to watch Goggins in training, noting that rules on media interviewing trainees or observing them have been tightened.

“As with all special warfare candidates and cadre, Goggins will not be subject to any public documentation consistent with our standard identity protection and operational security policies,” said Jen Gangemi, a spokeswoman for the special warfare wing. “He will be afforded the same discretion extended to every candidate in our pipeline and will not be made available for media interviews or coverage during training.”

The Air Force typically doesn’t accept applicants who are over 42 years old. Officials declined to comment specifically on how the  age requirement came to be waived for Goggins, who turned 51 in February.

“Given Master Sgt. Goggins’ background, it’s worth noting that the Air Force has welcomed special operators from other services to cross train into special warfare for decades,”  said Salitore, the special warfare wing executive officer. “Airmen entering the pipeline from sister services must meet the same standards and complete the same rigorous training as all other candidates.

“We value the experience gained from former Special Forces, Rangers, Marine Special Operations and SEALS that have joined our Air Force Special Warfare units once they have completed their training.”

Pararescue is a storied Air Force career. Pararescue candidates enter a 71/2-week special warfare candidate course at Lackland after basic military training. They run, ruck and swim extensively before moving to a four-week special warfare assessment and selection course, where they either make the grade or move on to another career field.

If they pass those tests, the airmen enter a four-week pre-dive course that includes intense calisthenics, middle- and long-distance running, swimming and water-confidence training.

This article originally published at Fitness guru David Goggins in S.A., training for Air Force pararescue program.