The 5-foot-8, 260-pounder estimates he has finished 276 triathlons since 1983, causing his doctor to marvel at this remarkable combination of fitness and admitted obesity.
"I am fat,'' said Alexander, 55. "I was born a big boy, and I'm always going to be big. But I'm healthy.''
Like many men his age, Alexander's silver hair is thinning. His bright blue eyes are going bad, and his barrel stomach is getting bigger. Other triathletes often mistake him for a race organizer.
"I'm a great bar bet,'' he said, laughing. "I don't look like I can walk across the street, let alone run a triathlon.''
Steven Blair, senior editor of the Surgeon General's Report on Physical Activity and Health, said a surprising number of people are both fit and obese.
In a recent study of obese men, Blair said that 45 percent had zero or only one of the major risk factors for an early death smoking, bad eating habits, a sedentary lifestyle, a history of heart disease. These men, despite their obesity, had no increased mortality rate.
"Most people see an obese person walking down the street and they think, 'This guy's a time bomb.' It's not necessarily so,'' Blair said.
Alexander is living proof.
In a recent super-triathlon in Eastern Hungary, he swam 9.6 miles, cycled 448, and ran what he calls "a 104.8-mile Bataan death march.''
Alexander attributes his great shape to persistence.
He said he competes at a furious pace, sometimes completing two triathlons in a week. He sleeps about 4 1/2 hours a night so he can put in the long hours he needs to train and conduct business at the oil company he co-owns.
Dr. Craig M. Phelps, his doctor for 15 years, called Alexander "one in a million.''
"And I say that statistically,'' Phelps said. "I haven't known of anyone his size who can do the swimming, the running, the cycling, at the ultra-distances Dave competes at. To be able to put all those things together at Dave's size is truly unique.''