The rat study may provide only small amount of evidence that running is actually addictive in humans, but exercise experts didn't need much convincing anyway.
"It's not across the board that it's addictive, but it can be addictive," said Wayne Westcott, director of the exercise science program at Quincy College in Quincy, Mass. "Those are the people who over-train, they have bad feet, surgeries, or injuries and they can't stop."
Former professional runner Per Kristian Moerk agreed that brain chemistry could get people addicted to running. But that's only part of the picture, he said.
"I think the other part of why running is addictive is the feedback," said Moerk, manager for the sports medicine center at Wake Forest University Baptist Medical Center.
Moerk noticed runners also run to keep their bodies slim, have more energy during the day and for old-fashioned competition.
"You look better, your self-esteem improves. You get compliments," said Moerk.
But if running is so addictive, it begs the questions of what happened to all the rest of us who hate the tedium of the treadmill with a passion.
"In most things in life, and the things we research, there are responders to almost everything and there are non-responders," said Westcott. "Unfortunately, most people are not responsive to exercise and they don't seem addicted to exercise."
Indeed, 95 percent of us seem quite lazy. A 2003 study by the National Health and Nutrition Examination Survey (NHANES) outfitted 10,000 people across the United States with an accelerometer and tracked how long and how fast they moved during the day.
The results showed that less than 5 percent of adults older than 20 get at least 30 minutes of physical activity five days a week.
"That included activities of daily living, like working in the garden, and they could even do it in 10-minute segments," said Westcott.
According to Westcott, the physical activity standard in the NHANES trail amounted to walking 2.5 miles per hour, which is not even a brisk walking pace.
"Very few people are addicted, however the other 5 percent of us can get quite addicted to exercise," said Westcott. "I could cut off my leg and I'd still want to go exercise."
As for Jensen, he said his days of ultra marathons are dwindling.
"I lost motivation. I had accomplished what I dreamed of doing, and to me it was quite a feat. I didn't want to find something harder, I didn't want to beat myself," said Jensen. "Now I try to run one ultramarathon a year, just to be active."