"I DON'T ENJOY MY JOB," SAYS DEFENSE SECRETARY ROBERT GATES. "BEING THE SECRETARY OF WAR IN A TIME OF WAR IS A VERY PAINFUL THING," HE TELLS KATIE COURIC -- "60 MINUTES"

The job of wartime defense secretary is not a job anybody should like says the man holding it right now, Robert M. Gates, in a candid and wide-ranging interview with Katie Couric. Couric spoke numerous times to Gates when she accompanied him on a trip to Egypt, Saudi Arabia and Afghanistan last week, representing the most access that Gates has given to a television reporter. The segment, videotaped mostly in Afghanistan where Gates is refocusing America's war there, will be broadcast on 60 MINUTES Sunday, May 17 (7:00-8:00 PM, ET/PT) on the CBS Television Network.

"The truth of the matter is, being secretary of war in a time of war is a very painful thing....How can you like a job when you go to Walter Reed and you know you sent those young men and women in harm's way?" he asks Couric. "Every single person in combat today I sent there and I never forget that for a second. I don't enjoy my job." Click here to watch an excerpt.

Gates has now held the crucial cabinet post in two successive administrations, serving George W. Bush and now Barack Obama - the only defense secretary ever invited to continue his service by a new administration of the opposite party. Why did he stay on? "Because it's my duty and I do it almost exclusively for these young men and women in uniform out there," he says. "Whatever I can do to help them, the rest is all fluff as far as I'm concerned."

Asked to describe the two presidents under whom he served, Gates chooses his words carefully. On Bush, he says, "Committed, questioning and eager to make a decision and move on." Of Obama, Gates says, "Deliberative, decisive and calm."

Gates is winding down the war in Iraq and ramping up the one in Afghanistan. He's adding thousands of troops in Afghanistan to create a surge like the one in Iraq credited with stabilizing that country. The Afghan Army and police can then take over in theory, once the populace feels safe from Taliban terror. But how long before U.S. troops can come home? "What it would take is the Afghan Army growing and doing its job well. It would take the effectiveness of our own strategy and our own forces. It would take bringing better governance to the country," he tells Couric. "It would take a lot of different things to have a finite time when we can say, �We're out of here.'"

Gates says it would take two to four years for the Afghan Army to begin taking the lead in military operations. At the rate of about 20,000 new soldiers a year, it will take years before the Afghan Army gets the 160,000 additional soldiers most analysts estimate the country will need to maintain control.

Pressed for the shortest timetable for a U.S. troop pullout, Gates says, "You want certitude where there is no certainty. This is a war."