So while health-care workers might have been surprised by the sexagenarian's tenacity, friends and fellow runners knew Green would win the fight for his life.
Green, from Columbia, Maryland, got his start in running much as many young men of his generation did, as a football player wannabe in high school in northern Illinois. "I was too small for football, but my brother told me about cross-country, and I decided to give it a try," he said. "I hated it at first, but eventually I began improving my times and learned to like it."
Green had aspirations of running at Concord College in West Virginia. But he was told by the coach that his hair was too long and his mustache too radical, so he took a break from formal training. After college, he heard about a 10-kilometer event organized by the Howard County Striders running club and ran it.
"Around 1983, I read about a 100-miler in a magazine," Green said. "All the pictures showed smiling, happy runners, so I thought, 'How hard can it be?' "
Green registered for Virginia's Old Dominion 100-miler and quickly learned the answer: very hard. "It was my first time on trails, and I made it to Mile 60 before the race organizers made me drop out due to dehydration," he said. "I left there with no intention of ever trying that again, but I kept replaying the race in my mind and decided I had to go back."
He did - and finished - the following year. Around that time, another article caught his attention - one about a fellow ultra runner attempting to run the four grand slam events, but over the course of several years. "It sounded like a nice adventure, so I decided to try all four in one season," Green said.
The four races - Old Dominion, Western States 100 in California, Leadville 100 in Colorado and Wasatch Front 100 in Utah - all took place between June and September. "Old Dominion and Western States were only two weeks apart," Green said, "and then I had a couple of months off before trying Leadville and Wasatch Front, which were also two weeks apart."
At age 35, Green completed all four. In the ensuing years, Green would run nearly 300 ultramarathons (distances of more than 26.2 miles), 50 of which were 100-milers. At his peak in the 1980s and 1990s, he even bagged a few wins.
In 2014, at age 64, Green decided to give the grand slam one more try. By this time, the Old Dominion race had been replaced with the Vermont 100.
Slower but no less determined, Green completed three of the four races under the cutoff time of 30 hours, some of them within minutes. But on the fourth, the Wasatch Front, Green dropped out at Mile 35 with crippling back pain. "It turned out I had a stress fracture in my sacrum," he said. "It was hard for me to stop, but I physically couldn't continue."
The following April, Green suffered an accident. A contractor by trade, Green was in his back yard trimming tree limbs with a friend. "The first branch we cut was uneventful," he said. "But the second branch bounced up and hit me in the back of the head."
Green's injuries were extensive: a fractured skull, a cut to the carotid artery, internal bleeding and subsequent strokes. For two weeks, his future remained uncertain, and physicians brought him into and out of a coma. The lasting impact included a damaged vestibular system, hearing loss and impaired vision.
After about a month, Green began working to regain whatever abilities he could. "Once I got home, I started taking walks outside with a walker," he said. "Within two months of the accident, I could walk a mile, but it was slow."
Returning to running remained a big question mark. "I wanted to go faster than I could with a walker," Green said. "I thought of using a baby jogger to help with the balance issues, and gave it a try."
Green's determination led to a long, gradual return to running. "It was very awkward, and it took more energy than it did before," he admitted. "But the accident took so much, and I didn't want to give up running - it defined me for so many years."
As he clawed his way back, the running community cheered him along - dedicating races to him, running club events beside him and keeping tabs on his progress.
Exactly a year after his accident, Green lined up with his baby jogger at North Carolina's Umstead 100 race with the intent of running 50 miles. "I made it in 15 hours, which gave me the idea to try another 100 with the standard cutoff of 30 hours," he explained.
Green selected the Yeti 100 in Abingdon, Virginia, because it is held largely on crushed limestone and dirt, which makes it easier than some others, and because it features a net drop of 2,000 feet. He didn't make the cutoff that year, mostly because he didn't have enough lights on his baby jogger to run safely after dark. With his balance still off, Green is especially dependent on his vision, and darkness limits that ability.
He returned last year with extra lights strapped onto the jogger. His longtime friend and running partner Charlie Romanello flew in from Los Angeles to accompany him. With just 15 minutes to spare, Green and Romanello crossed the finish line, pulling off the ultimate comeback.
Reflecting on his return to 100-milers, Green admits the experience is bittersweet. "In the old days, I might have won that race, and now I come in last place," he said. "It's not the way I wanted to return. But I was not going to let my injury defeat me."