"It's fun when you're done with it, not while you're doing it," Kelly said Wednesday, barely a week after being named to the marathon flight along with Russian cosmonaut Mikhail Kornienko.
The mission, set to begin in 2015, is intended as a medical test bed for even longer Mars expeditions in the decades ahead.
Space station life can be routine, Kelly noted during a news conference.
"In the morning, you wake up, you're at work. When you go to sleep, you're also at work. So imagine being in your office for a whole year and you never get to leave," he said. "That is a challenge and it presents its own set of issues, but I think I'm up for it."
As for being off the planet for that long, Kelly said he already knows how he reacts to horrific news while in orbit.
During his five-month space station mission that spanned 2010 and 2011, his sister-in-law, former Rep. Gabrielle Giffords, was shot in an assassination attempt in Tucson, Ariz. She is married to Kelly's identical twin, Mark, who retired as a NASA astronaut last year.
"Certainly, nothing good comes out of anything like this. But as a result, I do know how I respond to something along those lines," he said.
Kelly, 48, a Navy captain, has two daughters from a previous marriage, ages 9 and 18. The youngest, Charlotte, screamed "awesome" when she learned her father was selected for the one-year mission.
Brother Mark was all for it. So was Giffords. When Mark told his wife, she said, "a year in space, that's great," Scott Kelly said in a phone interview with The Associated Press.
Meanwhile, in Russia, Kornienko's wife wept at the news. The 52-year-old cosmonaut, a rocket engineer with one daughter, said he initially had some doubts about taking on such a challenge. He previously spent six months in space.
"A year is a serious time," Kornienko said in Russian. But he said his doubts did not last long, "and actually it was my initiative."
Kelly was among four astronauts on NASA's short list for the assignment. Each had served as a commander aboard the space station, and was able to perform spacewalks and robot arm operations. Medical information also went into the selection: A crew member could not have exceeded his limit for exposure to cosmic radiation, for instance.
Kelly said he has no idea how or why he ended up being chosen. He will set a U.S. space endurance record with this mission. No American has spent more than seven months in space at a time.
Russia, on the other hand, already has experience with yearlong space travel. But it's limited to the old Mir space station and more than a decade has passed.
Four Russian cosmonauts have spent at least one uninterrupted year in space. Another two came close.
The world record—14 months in a single mission—is held by Dr. Valery Polyakov.
"They all are alive and well today. Their health status is quite good for their age," said Dr. Igor Ushakov, director of the Institute for Biological Problems in Moscow.
Ushakov warned that the medical risks will be at least double what they are on the more typical six-month mission.
NASA space station program scientist Julie Robinson expects the two men to come back just fine. They will watch an assortment of multinational crews come and go during their tenure; up to six people live on the orbiting outpost at any one time.
The loss of bone mass is not nearly the problem it used to be in weightlessness because of improved exercise equipment and procedures, she said. The newest concern is impaired vision related to pressure on the brain and spinal cord; in some cases, astronauts suffer vision problems long after their flight.
In a chart held up by the director of Russia's piloted space program, Alexey Krasnov, nearly half the slots were red, indicating medical risks to eventual trips to the moon, asteroids and Mars.
"There are many things we don't know," Krasnov said. "We should take some risks upon ourselves" now before embarking on such ambitious endeavors beyond Earth's orbit.